poly_basis_dx     258 magick/distort.c     default:  return( poly_basis_dx(n-1,x,y) ); /* weird but true */
poly_basis_dx    2520 magick/distort.c               du.x += poly_basis_dx(k,d.x,d.y)*coeff[2+k];
poly_basis_dx    2523 magick/distort.c               dv.x += poly_basis_dx(k,d.x,d.y)*coeff[2+k+nterms];