GRM-012 Hard constraint layer: phrase

A/an follows sound

IF an indefinite article precedes a singular noun phrase, THEN choose A before a consonant sound OR AN before a vowel sound ELSE retain.

Why the rule exists

The choice follows pronunciation rather than spelling.

Condition

The rule fires when this evaluates true. Paths marked below must be present in the fact base; the engine reports the rest rather than guessing them.

ALL
- determiner.type eq "indefinite_article"
- noun_phrase.initial_sound exists

Facts this rule reads

Fact paths read by GRM-012 and whether the analyser derives them
Path	Source
`determiner.type`	You supply it
`noun_phrase.initial_sound`	You supply it

Then

select — target determiner.form

Else

retain — target current_text, No triggering condition is present.

LAKA

Smallest sufficient intervention

Yes

Primary axes

Sources

The Blue Book of Grammar and Punctuation — Jane Straus; Lester Kaufman
Accessible grammar, punctuation, examples, and exercises.
English Grammar in Use — Raymond Murphy
Self-study explanations and practice for core English grammar.
Practical English Usage — Michael Swan
Advanced grammar and usage reference for learners and teachers.

The raw rule record

Verbatim from 07-core-grammar-rules.json. Also available at /v1/rules/GRM-012.

{
  "id": "GRM-012",
  "name": "A/an follows sound",
  "domain": "grammar",
  "layer": "phrase",
  "strength": "hard_constraint",
  "human_logic": "IF an indefinite article precedes a singular noun phrase, THEN choose A before a consonant sound OR AN before a vowel sound ELSE retain.",
  "when": {
    "all": [
      {
        "path": "determiner.type",
        "operator": "eq",
        "value": "indefinite_article"
      },
      {
        "path": "noun_phrase.initial_sound",
        "operator": "exists"
      }
    ]
  },
  "then": [
    {
      "action": "select",
      "target": "determiner.form",
      "by": "noun_phrase.initial_sound",
      "map": {
        "consonant": "a",
        "vowel": "an"
      }
    }
  ],
  "else": [
    {
      "action": "retain",
      "target": "current_text",
      "reason": "No triggering condition is present."
    }
  ],
  "unless": [],
  "because": "The choice follows pronunciation rather than spelling.",
  "diagnostics": [],
  "examples": {},
  "source_ids": [
    "BK-002",
    "BK-004",
    "BK-010"
  ],
  "laka": {
    "smallest_sufficient_intervention": true,
    "primary_axes": [
      "scope",
      "depth",
      "magnitude",
      "reversibility"
    ]
  }
}