All field keywords beginning with html_form_ are passed directly into the form hash.
Thus,
they are really defined in form.tt which ships with Gantry.
html_form_constraint specifies a Data::FormValidator constraint which will be checked with either AutoCRUD or CRUD.
The simplest constraints are regexes,
but anything Data::FormValidator understands will work.
Note that you need backticks around the value in the bigtop file,
but inside them you should have unquoted perl code.
To see an example, build:
bigtop -c example.bigtop all
Change to the newly created Kids directory and look in the form method in lib/Kids/GEN/Child.pm.
