[ESS] comint-delete-output including prompt

[Richard M. Heiberger]

On further looking I found that

C-c C-n   comint-next-prompt
C-c C-p   comint-previous-input

are both bound to the keys indicated above.  They work
in both iESS mode (*R* and *S* buffers) and in
ess-transcript-mode (myfile.rt and myfile.st).

To highlight and then remove a commandline/output region,
Do C-c C-p, C-@, C-c Cn, C-w

Rich