STK_HALFPINTL computes an intersection of lower half-planes

 CALL: [A, B, Z] = stk_halfpintl (A, B)

    computes the intersection of the lower half-planes defined by the vector
    of slopes A and the vector of intercepts B.  The output vectors A and B
    contain the slopes and intercept of the lines that actually contribute to
    the boundary of the intersection, sorted in such a way that the k^th
    element corresponds to the k^th piece of the piecewise affine boundary.
    The output Z contains the intersection points (shorter by one element).

 ALGORITHM

    The algorithm implemented in this function is described in [1, 2].

 REFERENCE

   [1] P. I. Frazier, W. B. Powell, and S. Dayanik.  The Knowledge-Gradient
       Policy for Correlated Normal Beliefs.  INFORMS Journal on Computing
       21(4):599-613, 2009.

   [2] W. Scott, P. I. Frazier and W. B. Powell.  The correlated knowledge
       gradient for simulation optimization of continuous parameters using
       Gaussian process regression.  SIAM J. Optim, 21(3):996-1026, 2011.