If a type is interpreted as a set of strings, then the ← {\displaystyle \leftarrow } may be interpreted as ⊇ {\displaystyle \supseteq \,\!} , that is, "includes as a subset".
「読んでよ（お）る」のように補助動詞としても用いる（「-ていよ（お）る」の略）。