Add failing test that demonstrates bug
Add JavaScript examples
Add bracket term to language.
Update drivers to use new bracket term for unspecified []
Reflect error message change
Just contain both opposing terms and call the right one
s/get_field/bracket/
Restructure bracket_term_t to only call args->arg once per arg
Record obsolete instructions and renumber BRACKET
Don't need this any more
Fix indentation and stray variable
Use lambdas here and clean things up
This doesn't need to be virtual
Alter error message to resemble that in check_type
Handle grouped data in nth.

The problem here is that get_field could cope with grouped data and nth
couldn't; so there was no coherent result for that method.  We resolve
the situation by changing nth to cope with grouped data.
Replace duplicated MAP code here with a fallthrough case.

The MAP code was the same as the FILTER code, which made me wonder if
something strange was happening at first glance.
Renumber BRACKET to 157, since INDEX_RENAME stole 156
Adjust to violation of UnorderedAssociativeContainer concept
Proof of concept showing that bracket_term_t need not subclass obj_or_seq_op_term_t.
Renumber BRACKET again.