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Virtually every programming environment has some formatted output API. At the very least, they must allow for messages like "%i bytes sent". Some environments, like PHP, are huge formatted output APIs. The printf family of functions is an example of a small but still immensely useful implementation. ABC adapts that as $printf.
Still, there is a need for a more sophisticated templated output API. For example, LEX code generation needs one. A common line of thinking is to make up a scripting language and make an interpreter for that. The other one is to interleave the code with template chunks using some bracket notation, then pre-compile that into normal code. Of course, there is no need to invent these things, as we can get something by invoking our favorite package manager. Still, that is a rather fat dependency for a low-level system library. It is also fun to consider that ABC needs it as a tool for the LEX/ragel parser generator combo. Chaining three code generators does not seem wise. Also, the task of string substitution is a modest one and ABC commandments tell us that the code should be small, simple, orthogonal and arbitrarily composable with the rest of ABC.
Long story short, it takes 150 lines to implement a basic templating engine on top of a plain ABC byte buffer. NEST stands for "nested templates". Yes, differently from PHP, bash and others, a variable can be substituted for a template that will be recursively subjected to further substitutions. Despite that NEST is a single-pass algorithm. Yes, minimalism.
