gf-core/grammars/multiple/SystemMathEng.gf

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) ;
} ;