+' Rules:
+' - Quantities [0-9]+ and charges [0-9]*[-+−] are recognized after an element
+' symbol [A-Z][a-z]? or a closing delimiter [])}] . Hyphens are converted
+' into real minus signs.
+' - A charge sign [-+−] is ignored if it is followed by a letter, digit,
+' opening delimiter, or [<>] . (Charges should appear only at the end of a
+' formula, and we want to avoid matching ordinary hyphens in text.)
+' - When digits followed by a charge sign are recognized, the last digit
+' becomes part of the charge and the remaining digits become the quantity.
+' (Charges rarely have absolute value more than 9.)
+' - Exception: If a single digit follows O or a closing delimiter, that digit
+' is always the quantity. (Handle NO3- and Fe(OH)2+. I think oxygen is the
+' only element that frequently has a quantity as part of a +/-1 ion. A group
+' is rarely parenthesized unless it has a quantity.)
+'