Rationale: Polytonic Greek input is of interest primarily to classicists, people who want to reproduce Ancient Greek quotations, and for input of Katharevousa Greek. There already exist several input methods for Polytonic Greek in the greek.el file distributed with Emacs, but this file improves over them in the following 2 ways:
1) Mapping based on "standard" Win and Mac layouts—The existing input methods' mappings are modeled after ad-hoc polytonic input schemes devised by classicists, in conjunction with specialized software. The mappings I use are modeled after Mac and Windows polytonic Greek keyboard mappings, (in turn modeled after monotonic Greek), and hence require less context-switching to use for people accustomed to modern and popular Greek keyboards.
2) Input of combining character sequences possible—While the existing input methods allow for the input of bare letters and precomposed letter+diacritics, but not for Unicode letter+diacritic sequences that are not represented by precomposed characters. For example, the sequence <alpha>+<combining macron>+<combining acute accent> is not represented by any precomposed character, but appears frequently in critical editions of classics. greek-polytonic.el allows for the input of combining characters themselves, and substitutes such sequences with their Unicode-canonical precomposed equivalents if they exist; hence input from this method satisfies Unicode-NFC (Normalization Form Canonical Composition), while allowing input of sequences that have no corresponding precomposed character. Though it is to be admitted that font support and Emacs's display support for such decomposed sequences is still rudimentary and the sequence may visually appear funky.
While I consider this package functionally complete, there are several avenues for further work when I have time and if there is interest:
—Allow "prefix" input of diacritics à la most other input methods. (currently only "postfix" diacritic input is supported)
—Allow input of greek numeral modifiers, of archaic letters, and of greek "symbols".
—Allow input of non-combining versions of diacritics.
Thanks for your attention and hard work.