Nested Delimited Constructs

There are two kinds of terminating delimiters:

postfix separators (TBD: currently only on arrays)

terminators

Both of these can be optional. (TBD: can postfix separators be optional? I believe there is no finalSeparatorCanBeMissing though there is a terminatorCanBeMissing property)

This means the parser can encounter a terminating delimiter from an enclosing array or group to indicate the termination of a nested array or group.

The behavior of parsing when it encounters one of these terminating delimiters, that is, one that is defined on an enclosing construct, is to indicate that it found a terminating delimiter of some sort, but to not consume that terminating delimiter when computing the new pos result. That is, the parse function succeeds and returns a new pos, value, etc., but the pos reflects the position where the terminating delimiter begins.

The invariant that this insures is that a parser for any construct consumes its own delimiters and only its own delimiiters.  

For example, the parse function for a sequence group which has separators specified will recursively parse the elements it contains; however, if one of those elements' representation is terminated by finding the enclosing sequence group's separator, then that separator will not be consumed, and when the recursive parse unwinds back to the parse function of the enclosing sequence group, the separator will then be consumed by the sequence group's parse function which is prepared to recognize it and advance past it.

This principle works regardless of how deeply nested the constructs are.

Parsing must, however, take into account the complete set of terminating delimiters that it might encounter, along with the escape/quoting schemes that can be specified for them which allow them to appear as content rather than as delimiters.