The off-side-rule (indentation-sensitive layout) algorithm Python's own tokenizer uses, and Haskell's/F#'s: given each logical line's indentation width, in order, maintain a stack of open indentation levels and emit :indent/:dedent markers as the width rises or falls -- so a parser downstream never has to know about columns at all, just consumes ordinary tokens.

This is deliberately a different strategy from Ichor's own Grammar.IR.Indent/@samecol (column-position checks embedded directly inside grammar rules as zero-width assertions) -- not a replacement for it, and not something Ichor's core needs, since it already made that different design choice. It exists for a grammar author porting a real language whose own reference implementation synthesizes INDENT/DEDENT pseudo-tokens at the lexer level (as Python's, Haskell's, and F#'s all do) and would rather mirror that approach directly than rewrite it as embedded column checks.

Deliberately unopinionated about what counts as a line worth indentation-checking at all (blank lines, comment-only lines, and lines continued inside brackets are all real, genuinely language-specific policy choices) -- a caller decides which lines to feed in and what each one's width is; this only implements the mechanical stack-based algorithm on top of that decision.