def tril(P, k=0):
    """Lower triangle of coefficients."""
    A = P.A.copy()
    for key in P.keys:
        A[key] = numpy.tril(P.A[key])
    return Poly(A, dim=P.dim, shape=P.shape)
def tril ( P , k = 0 ) : A = P . A . copy ( ) for key in P . keys : A [ key ] = numpy . tril ( P . A [ key ] ) return Poly ( A , dim = P . dim , shape = P . shape )
Lower triangle of coefficients.