- meta keys: ```mmt theory MetaKeyTest : ur:?LF = symbol: type ❙ usage: type ❘ meta ?symbol ?symbol ❙ // only `meta ?MetaKeyTest?symbol ?symbol` works ❙ ❚ ``` <s> - in includes: ```mmt theory OuterTheory = theory InnerTheory1 = ❚ theory InnerTheory2 = include ?InnerTheory1 ❙ ❚ // only `include ?OuterTheory?InnerTheory1` works ❙ ❚ ``` </s>