Need to write this...

A short description in place of a full one is that sanitizers are functions
which "sanitize" a descriptor. These descriptors are usually ones created by
calling a FieldType with some arguments. Sanitizers are specific to certain
kinds of FieldTypes, and are used to make sure that certain entries exist
in the descriptor. If they dont(or some combination of entries isnt valid)
the sanitizer will flag the BlockDef as bad and add an error message to
the error string.