add backslash rules (#126)

Author: shashi

src/simplify_rules.jl

@@ -45,6 +45,8 @@ let
         @rule(identity(~x) => ~x)
         @rule(-(~x) => -1*~x)
         @rule(-(~x, ~y) => ~x + -1(~y))
+        @rule(~x::_isone \ ~y => ~y)
+        @rule(~x \ ~y => ~y / (~x))
         @rule(~x / ~y => ~x * pow(~y, -1))
         @rule(one(~x) => one(symtype(~x)))
         @rule(zero(~x) => zero(symtype(~x)))