Useful to robustify prior to e.g. regexp substitution of
    code strings

def _dummify_expr(expr, basename, symbs):
    """
    Useful to robustify prior to e.g. regexp substitution of
    code strings
    """
    dummies = sympy.symbols(basename+':'+str(len(symbs)))
    for i, s in enumerate(symbs):
        expr = expr.subs({s: dummies[i]})
    return expr