Move witness output into separate interface

Not all incremental_goto_checkert implementations
can provide witnesses.

Author: peterschrammel

src/goto-checker/incremental_goto_checker.h

@@ -67,11 +67,7 @@ class incremental_goto_checkert
   /// Builds and returns the counterexample
   virtual goto_tracet build_error_trace() const = 0;
 
-  // Outputs an error witness in GraphML format (see `graphml_witnesst`)
-  virtual void output_error_witness(const goto_tracet &) = 0;
-
-  // Outputs a proof in GraphML format (see `graphml_witnesst`)
-  virtual void output_proof() = 0;
+  virtual const namespacet &get_namespace() const = 0;
 
 protected:
   incremental_goto_checkert(const optionst &, ui_message_handlert &);

src/goto-checker/witness_provider.h

@@ -0,0 +1,27 @@
+/*******************************************************************\
+
+Module: Interface for outputting GraphML Witnesses for Goto Checkers
+
+Author: Daniel Kroening, Peter Schrammel
+
+\*******************************************************************/
+
+/// \file
+/// Interface for outputting GraphML Witnesses for Goto Checkers
+
+#ifndef CPROVER_GOTO_CHECKER_WITNESS_PROVIDER_H
+#define CPROVER_GOTO_CHECKER_WITNESS_PROVIDER_H
+
+/// An implementation of `incremental_goto_checkert`
+/// may implement this interface to provide GraphML witnesses.
+class witness_providert
+{
+public:
+  // Outputs an error witness in GraphML format (see `graphml_witnesst`)
+  virtual void output_error_witness(const goto_tracet &) = 0;
+
+  // Outputs a proof in GraphML format (see `graphml_witnesst`)
+  virtual void output_proof() = 0;
+};
+
+#endif // CPROVER_GOTO_CHECKER_WITNESS_PROVIDER_H