def inexact(pa, pb, pc):
    """Direction from pa to pc, via pb, where returned value is as follows:

    left:     + [ = ccw ]
    straight: 0.
    right:    - [ = cw ]

    returns twice signed area under triangle pa, pb, pc
    """
    detleft = (pa[0] - pc[0]) * (pb[1] - pc[1])
    detright = (pa[1] - pc[1]) * (pb[0] - pc[0])
    det = detleft - detright
    return det