def aa_name(aa):
    """Given the single-letter code for an amino acid, find the
       corresponding name.  Throws an exception if given an 
       invalid code."""
    if not(is_string(aa)):
        parameter_error("aa_name", 1, "string", aa)
    if len(aa) != 1:
        parameter_error("aa_name", 1, "length-one string", aa)
    if not(aa in amino_acids):
        parameter_error("aa_name", 1, "one-character amino acid", aa)
    return aa_name_dict[aa]