weight   : compound         { $item[1] }
  compound : group(s)         { sum( @{$item[1]} ) }
  group    : element factor   { $item[1] * $item[2] }
  factor   : /\d+/            { $item[1] }
           |                  { 1 }
  element  : /[A-Z][a-z]*/    { $weights{ $item[1] } }
           | "(" compound ")" { $item[2] }