mainline filter matches the a position if the position depth of 0, that is, if it is a mainline position.
By default, all positions considered by CQL are mainline positions, so
mainline is always true. If CQL has variations set in the CQL header, or was invoked with the -variations option, then some positions might not be mainline positions.
mainline filter is often used with the echo filter to distinguish the main
theme idea from a fail in a variation: knightpawnforkecho.cql, queenpawnpinecho.cql, underpromotionecho.cql, wurzburg-plachutta.cql, zugzwang1.cql, zugzwang2.cql.