substitute included vars with abs path

[zmstone]

file a:

foo={x:1}{include b}
bar={x:2}

file b:

y: ${bar.x}

this causes {unresolved,['foo.bar.x']} exception.
according to the spec:

Substitutions in included files are looked up at two different paths; first, relative to the root of the included file; second, relative to the root of the including configuration.