the 2 gray parallel lines designate active boundaries
direction of the active boundary lines is parallel to
axis the current figure falls;
location of the active boundary lines is such that they
minimally include current array of settled figures;
when a figure settles down, it may drop one or more lines
if it completes those such that line occupies all space
between the active boundaries;
a line to drop may be longer then the distance between
active boundary lines, but it is important that the line
does not have breaks between the active boundaries;
when a figures settles down, only lines which include
blocks from the figure may be dropped; all other lines
remain the same even if they satisfy criteria to be
dropped;
