fix power of negative exponent

Author: shashi

src/direct.jl

@@ -43,6 +43,11 @@ function simplified_expr(O::Operation)
     isempty(O.args) && return O.op.name
     return Expr(:call, Symbol(O.op), simplified_expr.(O.args)...)
   end
+  if O.op === (^)
+      if length(O.args) > 1  && O.args[2] isa Constant && O.args[2].value < 0
+          return Expr(:call, :^, Expr(:call, :inv, simplified_expr(O.args[1])), -(O.args[2].value))
+      end
+  end
   return Expr(:call, Symbol(O.op), simplified_expr.(O.args)...)
 end