'Return the index of a dummy variable in the list of limits.
Usage
``index(expr, x)``  returns the index of the dummy variable ``x`` in the
limits of ``expr``. Note that we start counting with 0 at the inner-most
limits tuple.
Examples
>>> from sympy.abc import x, y, a, b, c, d
>>> from sympy import Sum, Product
>>> Sum(x*y, (x, a, b), (y, c, d)).index(x)
0
>>> Sum(x*y, (x, a, b), (y, c, d)).index(y)
1
>>> Product(x*y, (x, a, b), (y, c, d)).index(x)
0
>>> Product(x*y, (x, a, b), (y, c, d)).index(y)
1
See Also
reorder_limit, reorder, reverse_order'

def index(expr, x):

    variables = [limit[0] for limit in expr.limits]
    if (variables.count(x) != 1):
        raise ValueError(expr, 'Number    of    instances    of    variable    not    equal    to    one')
    else:
        return variables.index(x)
