private void setTarget()  {
    if (implicitTarget) {
      call(resources.getAstNodeCache().InteractionBuilder_AddEqualTarget, AstUtil.getImplicitParameterRef(activeWithOrMockClosure));
    } else {
      call(resources.getAstNodeCache().InteractionBuilder_AddEqualTarget, AstUtil.getInvocationTarget(call));
    }
  }
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