## Comet parameter: add_X_user_amino_acid

- This parameter allows users to define their own custom residue. Just encode the letter 'X' in the input FASTA file and specify its mass here.
- The letter 'X' has no default mass. So the mass entered here will be its residue mass.
- The default value is "0.0" if this parameter is missing. If any peptide contains the letter 'X' while this parameter value is set to 0.0, that peptide will not be analyzed.

Example:
`add_X_user_amino_acid = 100.8`