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);\n"]
    else:
        mutgamma = ["rateCatMut := 1.0;\n"]
    return mutgamma