def setGM(gm):
    """Set variable mutation rate, if `gm` is given."""
    if gm > 0:
        mutgamma = ['global shape;\n', 'category rateCatMut =(' + str(gm) +
            """, EQUAL, MEAN, GammaDist(_x_,shape,shape), CGammaDist(_x_,shape,shape),0,1e25);
"""
            ]
    else:
        mutgamma = ['rateCatMut := 1.0;\n']
    return mutgamma
