Canonical name for Iseq from "Implementing Functional Languages: a tutorial"

Question

In Simon Peyton Jones's "Implementing Functional Languages: a tutorial", a data structure for efficient string building called Iseq, also referred to as iseq and iseqRep. This can be found by searching through the document.

Is this Iseq name a canonical abbrevation for something? What does the "i" prefix refer to? I'm wondering this because I wonder if this is more widely used, and I can't find it anywhere in the literature.

I've been looking elsewhere for anything with the name, and the only eponymous structure is Clojure's ISeq, which is a very different thing. The two best guesses I have at the moment are:

• The "i" was completely arbitrary, and used as a prefix to identify all related functions.
• The "i" might stand for "indented sequence"? This is pretty much the only "i" that I could think of.