from sympy.simplify import trigsimp as sympy_trigsimp
from sympy.simplify.fu import TR10


def trigsimp_custom(self, **args):
    """
    Default trigsimp (in sympy >= 0.7.3) reduces sum of sin/cos products, for example

        trigsimp(-sin(x)⋅cos(y) + sin(y)⋅cos(x))
        >> -sin(x - y)

    We have to undo this step, which is what happens here.
    """
    return TR10(sympy_trigsimp(self, **args))