improper impeachment is the problem; as a subterfuge for improper purposes
i)    If W testifies to a non-collateral matter on direct, then opponent may impeach on both cross and by extrinsic E.
ii)    If W testifies to a collateral matter on direct, then opponent may impeach on cross, but must take the answer.
iii)    set-ups not allowed:  Setups or “smuggling” –> if W never testifies on direct to the fact to be impeached but cross-ex elicits facst on cross, for the purpose of setting the W up for contradiction, and then seks to contradict the answer by extinsic E