private ExpressionStatement register() {
    ExpressionStatement result = new ExpressionStatement(
      createDirectMethodCall(
        resources.getMockInvocationMatcher(),
        resources.getAstNodeCache().MockController_AddInteraction,
        new ArgumentListExpression(builderExpr)));
    result.setSourcePosition(stat);
    return result;
  }
If the given statement is a valid interaction definition, returns the rewritten statement.
If the given statement is not an interaction definition, returns null.
If the given statement is an invalid interaction definition, records a compile error
and returns null.
spock-core/src/main/java/org/spockframework/compiler/InteractionRewriter.java