programs in demo/ need a check phase

[rocky]

Demo programs need a check step before eval. My failing attempt to add it: https://gist.github.com/rocky/7970148

I get

 $ go run binaryexpr.go 
 # command-line-arguments
 ./binaryexpr.go:17: cannot use aexpr (type interactive.Expr) as type ast.Expr in >      function argument:
   interactive.Expr does not implement ast.Expr (missing ast.exprNode method)

So guess I'm not getting the changed definition in. (I did a go install previously)