Support function for iat().
    Find cutting time, when gammas become negative.

def _cut_time(gammas):
    """Support function for iat().
    Find cutting time, when gammas become negative."""

    for i in range(len(gammas) - 1):

        if not ((gammas[i + 1] > 0.0) & (gammas[i + 1] < gammas[i])):
            return i

    return i