Implement safe data and enter.

[Drup]

Following discussion #5. I really think it's better to hide unsafe combinators under the carpet, rather than creating a Safe module. Good defaults are important and user will not look further to use "the other combinators".

[Drup]

One thing I'm not satisfied is that it's still possible to place enter in a nonsensical way. I could enforce the proper placement of enter by typing, but it needs to change the API some more, and I'm not sure it's such an issue (the rule is quite simple and is explicit in the documentation).

[yminsky]

Having just stumbled into some of the same soundness issues, I'm interested in this patch. Any hope of getting it integrated? Having a safe API that one could stick to most of the time would be a lovely thing....