http://www.postgresql.org/docs/9.3/interactive/datatype-boolean.html also refers to it as `boolean` with three states though, t, f and u(nknown = sql NULL)
On the ancient server I tried, the type was actually `bool` (major version 6 o.O)' 
Shall we add two types to cover it? `bool` and `boolean`? On some very old version it is only `bool`, on 6+ its both, and on 7.5 upwards its only `boolean`