(datanames: (dm:'B','Y', 'A','X')) produces the array
Sometimes, you may wish to obtain some information about a datamap. You may want to list all of its data names, or determine how many entries it has. You can use the (datanames:) macro to do these things: if you give it a datamap, it produces a sorted array of all of its names. You can then (print:) them, check the length of the array, obtain a subarray, and other things you can do to arrays.