finish the idea behind #8, conditionally creating variables

by using space? , the ? makes for conditional creation of the variable
This is especially useful for constants (ie space)

test/risc/test_builder2.rb

@@ -25,6 +25,17 @@ module Risc
     def test_caller_reg
       assert_equal :Message , @builder.infer_type(:caller_reg).class_name
     end
+    def test_define_twice
+      @builder.caller_reg!
+      assert_raises{ @builder.caller_reg! }
+    end
+    def test_define_conditionally_first
+      assert_equal :r1 , @builder.caller_reg?.symbol
+    end
+    def test_define_conditionally_again
+      first = @builder.caller_reg!
+      assert_equal first , @builder.caller_reg?
+    end
     def test_caller_tmp
       assert_equal :Message , @builder.infer_type(:caller_tmp).class_name
     end