Comet parameter: add_C_cysteine
         
         - Specify a static modification to the residue C.
         
- The specified mass is added to the unmodified mass of C.
         
- The default value is "57.021464" if this parameter is missing.
         Note that in the example parameter file produced by Comet using the "-p"
         command line option, this parameter entry has a value of 57.021464 for
         cysteine carbamidomethylation.
         
Example:
         
add_C_cysteine = 57.021464