3.2.3 Intersecting
Intersecting two selections means that the resulting selection is, you
guessed it, the intersection of the two. Thus, if A is the set of
pixels defined by the first selection and B the set for the second,
is the result of intersecting the two. The intersection is
made by pressing the Shift and Control keys together before
pressing the left mouse button to begin the new selection. The
Control and Shift keys should be released before releasing the mouse
button. The result of intersecting an ellipse with the rectangular
selection in Figure
3.15(a) is shown in
Figure
3.15(d).
It is useful to note that pressing the Shift and Control keys
together while the mouse is in the image window has the effect of
changing the mouse cursor to an arrow with a 
sign below
it. This indicates that the next selection will be intersected with
any selection already existing in the image window.
