@Override
	public void prefixAlt(AltAST originalAltTree, int alt) {
		AltAST altTree = (AltAST)originalAltTree.dupTree();
		stripAltLabel(altTree);

		int nextPrec = precedence(alt);
		// rewrite e to be e_[prec]
		altTree = addPrecedenceArgToRules(altTree, nextPrec);
		String altText = text(altTree);
		altText = altText.trim();
		String altLabel = altTree.altLabel!=null ? altTree.altLabel.getText() : null;
		LeftRecursiveRuleAltInfo a =
			new LeftRecursiveRuleAltInfo(alt, altText, null, altLabel, false, originalAltTree);
		a.nextPrec = nextPrec;
		prefixAndOtherAlts.add(a);
		//System.out.println("prefixAlt " + alt + ": " + altText + ", rewrite=" + rewriteText);
	}
Tokens from which rule AST comes from
tool/src/org/antlr/v4/analysis/LeftRecursiveRuleAnalyzer.java