compiler.cfg.checker: new vocabulary for checking CFG invariants

diff --git a/basis/compiler/cfg/checker/authors.txt b/basis/compiler/cfg/checker/authors.txt
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+Slava Pestov
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diff --git a/basis/compiler/cfg/checker/checker.factor b/basis/compiler/cfg/checker/checker.factor
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+! Copyright (C) 2009 Slava Pestov.
+! See http://factorcode.org/license.txt for BSD license.
+USING: kernel compiler.cfg.instructions compiler.cfg.rpo sequences
+combinators.short-circuit accessors ;
+IN: compiler.cfg.checker
+
+ERROR: last-insn-not-a-jump insn ;
+
+: check-basic-block ( bb -- )
+    peek dup {
+        [ ##branch? ]
+        [ ##conditional-branch? ]
+        [ ##compare-imm-branch? ]
+        [ ##return? ]
+        [ ##callback-return? ]
+        [ ##jump? ]
+        [ ##call? ]
+        [ ##dispatch-label? ]
+    } 1|| [ drop ] [ last-insn-not-a-jump ] if ;
+
+: check-cfg ( cfg -- )
+    entry>> reverse-post-order [
+        instructions>> check-basic-block
+    ] each ;
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