Two fixes:

1. When there was split of a multiscripts for chemistry, it didn't get put back together properly. Need to be careful at start of line.
2. For atomic numbers, give more credit for the mass number if it is withen a plausable range (from searches, "nuclear drip line" estimated values based on atomic number).

Author: NSoiffer

src/chemistry.rs

@@ -458,7 +458,7 @@ pub fn scan_and_mark_chemistry(mathml: Element) -> bool {
             }
         }
     }
-    debug!("...after marking:\n{}", mml_to_string(child));
+    // debug!("...after marking:\n{}", mml_to_string(child));
 
     if child.attribute(CHEM_FORMULA).is_none() && child.attribute(CHEM_EQUATION).is_none() {
         if !has_maybe_chemistry(mathml) {
@@ -590,48 +590,52 @@ fn is_changed_after_unmarking_chemistry(mathml: Element) -> bool {
         // could be no preceding children to canonicalization creating mrows (see issue #303), so might need to use parent, etc
         while preceding_children.is_empty() {
             preceding_children = parent.preceding_siblings();
-            parent = get_parent(parent);
             if name(parent) == "math" {
-                panic!("is_changed_after_unmarking_chemistry: error no preceding children to merge. mathml=\n{}", mml_to_string(mathml));
+                break;  // consider {SIN}^{-1} -- no preceding child
             }
+            parent = get_parent(parent);
         }
 
-        // deal with the first element (if it needs unwrapping, it has only prescripts)
-        let first_element_of_split = as_element(preceding_children[preceding_children.len()-1]);
-        // debug!("first_element_of_split: \n{}", mml_to_string(first_element_of_split));
-        if name(first_element_of_split) == "mmultiscripts" {
-            // take the base and make it the first child of preceding_children (what will get merged)
-            // put the rest of the elements (the prescripts) at the end of the parent last element (mathml) which must be an mmultiscripts
-            let first_element_children = first_element_of_split.children();
-            assert_eq!(name(mathml), "mmultiscripts");
-            let mut script_children = mathml.children();
-            assert_eq!(name(as_element(script_children[0])), "mi");
-            assert!(!script_children.len().is_multiple_of(2));  // doesn't have <mprescripts/>
-            script_children.push(first_element_children[1]);    // mprescripts
-            script_children.push(first_element_children[2]);    // prescripts subscript
-            script_children.push(first_element_children[3]);    // prescripts superscript
-
-            let base_of_first_element = first_element_children[0];  // base
-            assert_eq!(name(as_element(base_of_first_element)), "mi");
-            let script_base = as_element(script_children[0]);
-            let mut merged_base_text = as_text( as_element(base_of_first_element)).to_string();
-            merged_base_text.push_str(as_text(script_base));
-            script_base.set_text(&merged_base_text);
-            script_base.remove_attribute("mathvariant");
-            script_base.remove_attribute(ADDED_ATTR_VALUE);
-            script_base.remove_attribute(MAYBE_CHEMISTRY);
-            script_base.remove_attribute(SPLIT_TOKEN);
-            mathml.replace_children(script_children);
-    
-            first_element_of_split.remove_from_parent();
-            return true;
+        let mut new_script_children = vec![];
+        if !preceding_children.is_empty() {
+            // deal with the first element (if it needs unwrapping, it has only prescripts)
+            let first_element_of_split = as_element(preceding_children[preceding_children.len()-1]);
+            // debug!("first_element_of_split: \n{}", mml_to_string(first_element_of_split));
+            if name(first_element_of_split) == "mmultiscripts" {
+                // take the base and make it the first child of preceding_children (what will get merged)
+                // put the rest of the elements (the prescripts) at the end of the parent last element (mathml) which must be an mmultiscripts
+                let first_element_children = first_element_of_split.children();
+                assert_eq!(name(mathml), "mmultiscripts");
+                let mut script_children = mathml.children();
+                assert_eq!(name(as_element(script_children[0])), "mi");
+                assert!(!script_children.len().is_multiple_of(2));  // doesn't have <mprescripts/>
+                script_children.push(first_element_children[1]);    // mprescripts
+                script_children.push(first_element_children[2]);    // prescripts subscript
+                script_children.push(first_element_children[3]);    // prescripts superscript
+
+                let base_of_first_element = first_element_children[0];  // base
+                assert_eq!(name(as_element(base_of_first_element)), "mi");
+                let script_base = as_element(script_children[0]);
+                let mut merged_base_text = as_text( as_element(base_of_first_element)).to_string();
+                merged_base_text.push_str(as_text(script_base));
+                script_base.set_text(&merged_base_text);
+                script_base.remove_attribute("mathvariant");
+                script_base.remove_attribute(ADDED_ATTR_VALUE);
+                script_base.remove_attribute(MAYBE_CHEMISTRY);
+                script_base.remove_attribute(SPLIT_TOKEN);
+                mathml.replace_children(script_children);
+        
+                first_element_of_split.remove_from_parent();
+                return true;
+            }
+            new_script_children.push(ChildOfElement::Element(first_element_of_split));
         }
+        debug!("mathml after handling preceding children:\n{}", mml_to_string(mathml));
         let mut children_of_script = mathml.children();
         let split_child = as_element(children_of_script[0]);
-        let mut new_script_children = vec![ChildOfElement::Element(first_element_of_split)];
         new_script_children.append(&mut children_of_script);
         mathml.replace_children(new_script_children);     // temporarily has bad number of children 
-        // debug!("After making bad script:\n{}", mml_to_string(mathml));
+        debug!("After making bad script:\n{}", mml_to_string(mathml));
         if let Err(err) = merge_element(split_child) {
             panic!("{}", err);
         }
@@ -1370,7 +1374,7 @@ pub fn likely_adorned_chem_formula(mathml: Element) -> isize {
 
     let mut empty_superscript = false;
     if tag_name == "msup" || tag_name == "msubsup" {
-        // debug!("likely_adorned_chem_formula: mathml\n{}", mml_to_string(mathml));
+        debug!("likely_adorned_chem_formula: mathml\n{}", mml_to_string(mathml));
         let superscript = as_element(children[if tag_name == "msup" {1} else {2}]);
         empty_superscript = name(superscript) == "mtext" && as_text(superscript).trim().is_empty();
         if !empty_superscript {
@@ -1411,25 +1415,30 @@ pub fn likely_adorned_chem_formula(mathml: Element) -> isize {
             if is_adorned_electron(children[0], prescripts) {
                 return 100;     // very likely chemistry
             }
-            
+            let base = as_element(children[0]);
+            let base_name = name(base);
+            let atomic_number = if matches!(base_name, "mi" | "mtext") &&
+                                        let Some(atomic_number) = CHEMICAL_ELEMENT_ATOMIC_NUMBER.get(as_text(base)) {
+                        *atomic_number
+                    } else {
+                        return NOT_CHEMISTRY;
+                    };
             if pre_superscript_name == "mo" {
                 // Lewis dot prescript case
                 if pre_subscript_name != "none" {
                     return NOT_CHEMISTRY;
                 }
                 likelihood += likely_chem_superscript(pre_superscript);
             } else if pre_superscript_name == "mn" { // must have a pre-superscript (neutrons + protons)
-                // fix could make sure they are integers
-                likelihood += 1;        // looking like an atomic number                
-                if pre_subscript_name == "mn" {
-                    // make sure the atomic number matches the base
-                    let base = as_element(children[0]);
-                    let base_name = name(base);
-                    if (base_name == "mi" || base_name == "mtext") &&
-                        let Some(atomic_number) = CHEMICAL_ELEMENT_ATOMIC_NUMBER.get(as_text(base)) &&
-                            as_text(pre_subscript) == atomic_number.to_string() {
-                                likelihood = CHEMISTRY_THRESHOLD;
-                            }
+                if let Some(mass) = as_text(pre_superscript).parse::<u32>().ok() {
+                    // "drip line" is 1.5 * mass < 3.5 * mass -- it is possible to outside of this range, but VERY unlikely
+                    // to avoid floating point, we multiply by 2 and compare to 3 and 7
+                    if 3*atomic_number < 2*mass && 2*mass < 7*atomic_number {
+                        likelihood += 3;
+                    }
+                }
+                if pre_subscript_name == "mn"  && as_text(pre_subscript) == atomic_number.to_string() {
+                        likelihood = CHEMISTRY_THRESHOLD;
                 }
             } else {
                 return NOT_CHEMISTRY;
@@ -1476,6 +1485,7 @@ pub fn likely_adorned_chem_formula(mathml: Element) -> isize {
         likelihood += likely_chem_formula(base);
     }
 
+    debug!("returning from likely_adorned_chem_formula: likelihood={}, mathml\n{}", likelihood, mml_to_string(mathml));
     return likelihood;
 
 
@@ -2803,14 +2813,14 @@ mod chem_tests {
           </mrow>
         </mrow>
       </math>";
-        let target = "<math>
-            <mmultiscripts data-previous-space-width='-0.083'>
-            <mi mathvariant='normal'>U</mi>
-            <mprescripts></mprescripts>
-            <none/>
-            <mn>238</mn>
+        let target = " <math>
+            <mmultiscripts data-previous-space-width='-0.083' data-chem-formula='5'>
+                <mi mathvariant='normal' data-chem-element='2'>U</mi>
+                <mprescripts></mprescripts>
+                <none></none>
+                <mn>238</mn>
             </mmultiscripts>
-        </math>";
+         </math>";
         assert!(are_strs_canonically_equal(test, target, &[]));
     }
 
@@ -3024,15 +3034,15 @@ mod chem_tests {
         </mrow>
       </math>";
     let target = "<math>
-        <mrow data-chem-formula='7'>
-            <mmultiscripts data-previous-space-width='-0.083' data-chem-formula='3'>
+        <mrow data-chem-formula='11'>
+            <mmultiscripts data-previous-space-width='-0.083' data-chem-formula='5'>
                 <mi mathvariant='normal' data-chem-element='2'>O</mi>
                 <mprescripts></mprescripts>
                 <none></none>
                 <mn>18</mn>
             </mmultiscripts>
             <mo data-changed='added' data-chem-formula-op='0'>&#x2063;</mo>
-            <mmultiscripts data-previous-space-width='0.027999999999999997' data-chem-formula='3'>
+            <mmultiscripts data-previous-space-width='0.027999999999999997' data-chem-formula='5'>
                 <mi mathvariant='normal' data-chem-element='2'>O</mi>
                 <mprescripts></mprescripts>
                 <none></none>