[add Let and MkCont to modifyTails
John Meacham <john@repetae.net>**20060815065534] hunk ./Grin/Grin.hs 749
+    f lt@Let {expBody = body } = lt { expBody = f body }
+    f lt@MkCont {expLam = lam, expCont = cont } = lt { expLam = g lam, expCont = g cont }