A program served by ai_serve() (or by functai in any language), used
like a local one: its interface is the server's (GET /interface), its
inputs are bound and checked here before anything is sent, and what comes
back is checked against its outputs. Each call is logged here (program.kind
"remote") and there; the server's record names this call as its parent,
so the two logs make one call tree. It runs over a column and evaluates as
a local program does.