Avoids usefulFact recursion with nil parameter. Fixes #518.
Merge pull request #519 from gradha/pr_guards_against_nil_ast

Avoids usefulFact recursion with nil parameter. Fixes #518.
Adds some examples to JoinPath and / procs.
Explains parseHex initialization quirk.
Warns to not modify string findAll is iterating over.
Documents copyFile not copying attributes on posix.
Merge pull request #524 from gradha/pr_misc_docs

Miscellaneous doc tweaks