introducing multiple inheritance

grammars/multiple/SystemMathEng.gf

@@ -0,0 +1,6 @@
+concrete SystemMathEng of SystemMath = SystemEng, MathEng ** open
+  Prelude in {
+  lin
+    RSum x y d = 
+      ss (["the sum of"] ++ x.s ++ "and" ++ y.s ++ "is" ++ d.s) ;
+} ;