[x] Explain restrictions on [ ] and specifically how this is used to parse everything between them as a single element; used for feature arrays, but are permitted for variable names also.
[x] Be more explicit about transformations involving the same number of elements, and variables changed to other variables.
[x] Describe restriction on variable definitions: an entry must being either a sequence or a variable
[x] Elaborate on use of 0 and that it cannot be used in combination with other symbols, e.g.0h & and make sure it says "literal zero" characer
General improvements:
[ ]and specifically how this is used to parse everything between them as a single element; used for feature arrays, but are permitted for variable names also.0and that it cannot be used in combination with other symbols, e.g.0h& and make sure it says "literal zero" characer