brackets

The square brackets [ and ] have four functions in CQL

1. In a textual regular expression, to denote a class of characters:

   "mate in 29"~~[a-z ]+[0-9]+

2. In a [ and ] in piece designators, to denote a class of pieces or squares

   	♔[a1,b3]
   	♕[a-h1-5]
   	♕♖♗♘a2→♛♜♝♞][a1-8]

3. In string index to contain and index or range of indices into a string:

   	"stalemate"[3]
   	"mate"[1:5]

4. The [] symbol denotes the empty set:

   	x=[]