Something

exten => s,n,Read(subject|what-sub|3|||3)
exten => s,n,Gotoif($[${subject}=1]?gemara:mishna)

exten => gemara,1,NoOp()
exten => gemara,n(re-read),Read(masech|what-masech|3|||3)
exten => gemara,n,Gosub(masech-check,1)  ;sanity check
exten => gemera,n,GotoIf($["${GOSUB_RETVAL}" = "0"]?re-read)

exten => gemara,n(re-read-2),Read(daf|what-daf|3|||3|)   ; HERE i would like another sanity sheck using the same sub
exten => gemara,n,Gosub(masech-check,1)
exten => gemara,n,GotoIf($["${GOSUB_RETVAL}" = "0"]?re-read-2)

exten => gemara,n,ControlPlayback(custom/${LANGUAGE}/${masech}/${daf}|30000|3|1|0|5|8|j)

.
.
.

exten => masech-check,1,Noop(Check to see if masech exists)
exten => masech-check,n,Return(${IF($[${masech} => 28]?1:0)})