Adds TSet.isValid().
Adds TOrderedSet.isValid().
Moves TSet procs to their code block.
Adds test cases for remaining TSet procs.
Adds TOrderedSet.init(), wraps initOrderedSet around it.
Adds TSet.init(), wraps initSet around it.
Adds more docstrings to the sets module.
Factors common documentation in fragment to avoid repetition.
Adds definition of card term to sets module.
Merge pull request #1414 from gradha/pr_sets_improvements

Sets improvements