<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def where(cond, a, b, use_numexpr=True):
 """ evaluate the where condition cond on a and b Parameters cond : a boolean array a : return if cond is True b : return if cond is False use_numexpr : whether to try to use numexpr (default True) """


    if use_numexpr:
        return _where(cond, a, b)
    return _where_standard(cond, a, b)