Sanity-checking more arguments

[andrewcmyers]

Simple mistakes like passing the wrong kind of argument are going to give new users grief. Most user-facing methods should do run-time type checking of arguments. The run-time overhead would be paid at declaration time so should be insignificant.

[andrewcmyers]

line labels should be checked more

[andrewcmyers]

Also the checks themselves should be robust to null/undefined values