Added "fixated commodity pricing"

If you put '=' before an annotated commodity's price, it will cause any
future market valuation of that commodity to use that price, and ignore
whatever changes may have happened since in the market price.  This can
be useful if you are tracking gas expenses based on a standard rate
which, although it changes over time, should not adjust the historical
valuation of how much the gas cost at the time it was purchased:

    2009/01/01 Payee
        Expenses:Gas                100 GAL {=$2}
        Liabilities:MasterCard        $-200

src/amount.cc

@@ -546,6 +546,10 @@ amount_t::value(const bool		      primary_only,
       if (in_terms_of && commodity() == *in_terms_of) {
 	return *this;
       }
+      else if (is_annotated() && annotation().price &&
+	       annotation().has_flags(ANNOTATION_PRICE_FIXATED)) {
+	return (*annotation().price * number()).rounded();
+      }
       else if (optional<price_point_t> point =
 	       commodity().find_price(in_terms_of, moment)) {
 	return (point->price * number()).rounded();

src/commodity.cc

@@ -638,6 +638,12 @@ void annotation_t::parse(std::istream& in)
 	throw_(amount_error, _("Commodity specifies more than one price"));
 
       in.get(c);
+      c = peek_next_nonws(in);
+      if (c == '=') {
+	in.get(c);
+	add_flags(ANNOTATION_PRICE_FIXATED);
+      }
+
       READ_INTO(in, buf, 255, c, c != '}');
       if (c == '}')
 	in.get(c);
@@ -761,7 +767,10 @@ void annotated_commodity_t::write_annotations(std::ostream& out) const
 void annotation_t::print(std::ostream& out, bool keep_base) const
 {
   if (price)
-    out << " {" << (keep_base ? *price : price->unreduced()).rounded() << '}';
+    out << " {"
+	<< (has_flags(ANNOTATION_PRICE_FIXATED) ? "=" : "")
+	<< (keep_base ? *price : price->unreduced()).rounded()
+	<< '}';
 
   if (date)
     out << " [" << format_date(*date, string("%Y/%m/%d")) << ']';

src/commodity.h

@@ -363,8 +363,9 @@ struct annotation_t : public supports_flags<>,
 		      public equality_comparable<annotation_t>
 {
 #define ANNOTATION_PRICE_CALCULATED 0x01
-#define ANNOTATION_DATE_CALCULATED  0x02
+#define ANNOTATION_PRICE_FIXATED    0x02
-#define ANNOTATION_TAG_CALCULATED   0x04
+#define ANNOTATION_DATE_CALCULATED  0x04
+#define ANNOTATION_TAG_CALCULATED   0x08
 
   optional<amount_t> price;
   optional<date_t>   date;

test/baseline/feature-fixated-prices.test

@@ -0,0 +1,18 @@
+reg -V gas
+<<<
+2009/01/01 Payee
+    Expenses:Gas                100 GAL {=$2}
+    Liabilities:MasterCard        $-200
+
+2009/01/01 Payee
+    Expenses:Gas                100 FOO {$2}
+    Liabilities:MasterCard        $-200
+
+P 2009/03/01 12:00:00 GAL $3
+P 2009/03/01 12:00:00 FOO $3
+>>>1
+09-Jan-01 Payee                 Expenses:Gas                   $200         $200
+09-Jan-01 Payee                 Expenses:Gas                   $200         $400
+09-Mar-02 Commodities revalued  <Revalued>                     $100         $500
+>>>2
+=== 0