Formula with each discrete bonded residue or ion shown as a
separate moiety. See the CHEMICAL_FORMULA category description
for rules for writing chemical formulae. In addition to the
general formulae requirements, the following rules apply:
(1) Moieties are separated by commas ','.
(2) The order of elements within a moiety follows general rule
(5) in the CHEMICAL_FORMULA category description.
(3) Parentheses are not used within moieties but may surround
a moiety. Parentheses may not be nested.
(4) Charges should be placed at the end of the moiety. The
charge '+' or '-' may be preceded by a numerical multiplier
and should be separated from the last (element symbol +
count) by a space. Pre- or post-multipliers may be used for