tbl doesn't recognize 'z' column modifier

[g-branden-robinson]

This modifier is documented in Lesk's tbl paper in the Seventh Edition Unix manual (1979) and is supported by DWB 3.3 troff, upon which Heirloom Doctools is based.

$ cd ~/dwb
$ cat table-with-z-column-modifier.roff 
.TS
tab(@);
L Lz L.
one@/@two
.TE
$ DWBHOME=. ./bin/tbl table-with-z-column-modifier.roff | DWBHOME=. ./bin/nroff | cat -s
one   /  two

$ cd ~/heirloom
$ ./bin/tbl table-with-z-column-modifier.roff | ./bin/nroff | cat -s

table-with-z-column-modifier.roff: line 3: bad table specification character
tbl quits
 L.
one@/@two