The j d (calc-show-selections) command controls how
selected sub-formulas are displayed. One of the alternatives is
illustrated in the above examples; if we press j d we switch
to the other style in which the selected portion itself is obscured
by `#' signs:
3 ... # ___
(a + b) . . . ## # ## + V c
1* ............... 1* ---------------
. . . . 2 x + 1