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The goal of this ticket is to move phase-specific constructors to extension fields (Trees That Grow) - make illegal states unrepresentable.

• (done !2083 (closed)) HsWrap is specific to typechecking, should be inside XExpr.
• (done !2553 (closed)) ConPatIn and ConPatOut could become a single ConPat with fields depending on the pass.
• (done !2553 (closed)) Perhaps CoPat too.
• (in progress !3524)HsRecFld occurs only at the GhcRn phase.
• (in progress !3524 or !4782) HsRnBracketOut occurs only at the GhcRn phase.
• (in progress !4782) HsTcBracketOut occurs only at the GhcTc phase.
• (in progress !4782) AbsBinds occurs only at the GhcTc phase.
• HsTick and HsBinTick occur only at the GhcTc phase.
• HsConLikeOut occurs only at the GhcTc phase.
• HsSpliced could be moved to XSplice.
• Perhaps HsUnboundVar, HsIPVar, HsOverLabel which occur in GhcPs and GhcRn but not GhcTc.

(Initial idea mentioned by Simon and Richard during Zurihac)