What is procmail written in ?

Well, a friend and I are having this argument about what is procmail 
written in, I say it is an interface to some shell scripts. 

Alan 's page mentions something about it being a" paclage of fairly small
and efficient mail handling programs: procmail, formail, lockfile, etc "
that wasnt very descriptive etc. 

Shiraz

N.B I hope this isnt off topic, recently all my posts are off topic !