How about

    ;; rotateVertex [3dfxlib]
    ;;  Rotates a 3D vertex according to two angles.
    ;; Inputs:
    ;;  HL: Pointer to 3D vertex
    ;;  DE: Pointer to result
    ;;  C, B: X, Y angle
    ;; Notes:
    ;;  Each coordinate is two bytes long (elaborate here, please). Each angle is in [0, 255].