def addModification(self, aa,position, modMass, modType):
<EOL>if not modType:<EOL><INDENT>tmass = abs(modMass)<EOL>smass = str(tmass)<EOL>prec = len(str(tmass-int(tmass)))-<NUM_LIT:2><EOL>precFormat = '<STR_LIT:%>'+'<STR_LIT>'%prec<EOL><DEDENT>self.mods.add((aa,str(position),str(modMass),str(modType)))<EOL>
!!!!MODIFICATION POSITION IS 0 BASED!!!!!!
Modifications are stored internally as a tuple with this format:
(amino acid modified, index in peptide of amino acid, modification type, modification mass)
ie (M, 7, Oxidation, 15.9...)
such as: M35(o) for an oxidized methionine at residue 35