3.2.2 Subtracting
Subtracting a new selection from an existing one means that the result
is the first selection less the pixels contained in the intersection
of the two. Thus, if A is the set of pixels defined by the first
selection and B is the set for the second,
is the
result of subtracting the second from the first, where 
is the
set intersection operator.
A subtraction is made by pressing the Control key before
beginning the new selection. Once the new selection is begun, the
Control key should be released. The result of subtracting an ellipse
from the rectangular selection in Figure
3.15(a)
is shown in Figure
3.15(c). Note that it is the
new selection that is subtracted from the old selection, not vice
versa.
It is useful to note that pressing the Control key while the mouse is
in the image window has the effect of changing the mouse cursor to an
arrow with a minus sign below it. This indicates that the next
selection will be subtracted from any selection already existing in
the image window. Because I always forget which key controls addition
and which controls subtraction, I find the mouse cursor indicators are
particularly useful.
