To set a breakpoint from the Stepper, position the cursor where you want the breakpoint and choose
Stepper > Breakpoint
or click 
in the Stepper toolbar.
Figure 23.4 A breakpoint at the value of the inner call to
apply
When you step though the code, or choose Stepper > Continue , when a breakpoint is reached execution stops and the breakpoint is displayed.
In the example shown we have set a breakpoint at the value of the inner call to
apply
. so in this case, you can expand the top backtrace frame to display the value of that
apply
call.
To set a breakpoint from the Editor, see Breakpoints.