def tp(x0,U,lam=2,L=3):
    """
    tp means (t)ime to escape right with (p)ositive initial vel.
    """
    return ((2*(L - x0))/U + ((L - x0)*(L + x0)*lam)/U**2
            + L/(U + L*lam) - x0/(U + x0*lam))/3.