FHSelectIpol - selects which variables to interpolate in

       integer error, xt, xb

       subroutine FHSelectIpol(error, xt, xb)

       FHSelectIpol[xt, xb]

       FHSelectIpol  selects the interpolation mode for the trilinear coupling
       parameters.  Interpolation generally occurs  when  complex  values  are
       chosen  for MUE, M_3, At/Xt, or Ab/Xb, and FHSelectIpol chooses between
       interpolation in At vs Xt, and Ab vs Xb.

       error (output)
              zero if successful, otherwise the line  number  in  SelectIpol.F
              from which the error message was emitted.

       xt (input)
              whether to interpolate in At (0, default) or Xt (1).

       xb (input)
              whether to interpolate in Ab (0, default) or Xb (1).