def substitute_matrix(expr, subs=None):
    """
    Apply substitution method to matrix expression expr.

    Parameters
    ----------

    expr : pyphs matrix expression.
        Expression wich parameters are to be substitute.

    subs : dic
        Substitution dictionary '{symb : sub}'.

    Return
    ------

    expr_subs : pyphs matrix expression
        The resulting expression.
    """
    return expr.subs(subs)
