def expression_depth(expr):
    """Given an expression expressed as Python lists, uses recursion to return
    the depth of the expression, where depth is defined by the maximum number of
    nested operations."""
    max_depth = 0
    if type(expr) == list:
        max_depth = 1    
    for exp in expr:
        count = 0
        if type(exp) == list:
            count =  1 + expression_depth(exp)
        if count > max_depth:
            max_depth = count
    return max_depth