Issue 151: map:build

[michaelhkay]

Drops the proposed map:group-by function, replacing it with the more powerful map:build.

[michaelhkay]

For consistency, the default for the fourth parameter should be written fn:op(',') rather than op(','). The "fn" prefix should also be used with "concat" and "op" in the Notes section.

The formatting of the "more formally" equivalent expression has gone awry.

[michaelhkay]

I note that the old map:group-by() proposal said that if the key function returns an empty sequence, no entry is created in the index; the new spec makes this a type error. This seems a useful capability and I propose we reinstate it.

[ndw]

Accepted at meeting 007.