mod180.hs:8:5: error:
    Couldn't match expected type ‘T’
                with actual type ‘main:Mod180_A.T’
    NB: ‘main:Mod180_A.T’ is defined at Mod180_A.hs:3:1-10
        ‘T’ is defined at Mod180_B.hs:3:1-10
    In the expression: x
    In an equation for ‘z’: z = x