You can use something like this:
*----------------------
Procedure iCallStack
LParameter pProgram
Local ia, i,retval
retval=""
ia=1
do WHILE LEN(program(ia)) # 0 and program(ia)#pProgram
ia=ia+1
enddo

for i=ia to 1 step -1
retval=iif(i=ia, program(i),retval+" <- "+program(i))
endfor

return retval