allow negating Value objects

lib/gnucash/value.rb

@@ -59,6 +59,11 @@ module Gnucash
       end
     end
 
+    # Negate a Value
+    def -@
+      Value.new(-@val, @div)
+    end
+
     # Multiply a Value object
     # +other+ should be a Numeric
     def *(other)

spec/gnucash/value_spec.rb

@@ -51,6 +51,12 @@ module Gnucash
       c.should == 1.1
     end
 
+    it "allows negating a Value object" do
+      a = Value.new("123/100")
+      b = -a
+      b.to_s.should == "-1.23"
+    end
+
     it "allows multiplying a Value by a Numeric" do
       a = Value.new("100/100")
       b = 12