[article Include Test
    [quickbook 1.6]
    [id include-test]
]

[include include-inc.quickbook]
[include:foo include-inc.quickbook]

[def __defined__]

[? __undefined__ [include:foo1 include-inc.quickbook] ]
[? __undefined__ [include:foo1 not-a-file.quickbook] ]
[? __defined__
  Just trying including in a conditional macro. [include:foo2 include-inc.quickbook]
  With some text around it.]