building an older version of a port?

[Richard L. Hamilton]

I've forgotten how to do this, can someone explain it? I've fetched the old 
version of the Portfile corresponding to the version that I want. Specifically, 
there are some problems thought to be due to bison 3.7 and later, so I want to 
build bison 3.6.4 (keeping 3.7.1, but deactivated).