Occurrence required:
n : exactly n ;
n - m : n through m ;
> n : greater than n ;
< n : fewer than n ;
default (blank) is > 0 ;
Any non-contradictory combination of the preceding values is also
allowed; for example "1, 3-7, 9, >11".

Matches the 'occurrence' condition with a provided maximum number of
RGroup attachments. Returns the valid occurrences (numeric) for these
two combined. If none found, returns empty list.
Example: if R1 occurs 3 times attached to some root structure, then
stating ">5" as an occurrence for that RGoupList does not make
sense: the example R1 can occur 0..3 times. Empty would be returned.
If the occurence would be >2, then 3 would be returned. Etcetera.

Parameters:

maxAttachments - number of attachments

Returns:

valid values by combining a max for R# with the occurrence cond.