The and filter is a binary infix filter that matches a position only if the filter to its left and the filter its right each match the position. Thus, if left and right are filters, then
   left and right 
matches a position exactly when the compound filter
   {left right}
matches the position. For example:
  power >5 and power <3
Whether to use and or a compound filter is a matter of taste. One difference is that the compound filter sometimes returns a value but the and filter never does.


The file movedblackpieceecho.cql uses and in the line
x==Mover and source:(x&_)
This particular expression is true only if the set variable x has the same value as Mover, and the x intersects the set of empty squares in the position represented by source (see the : operator for a description of the meaning of the : here).