Substitution function

dynamic Substitution()

Add a substitution to a feature for a given script and language. @param {string} feature - 4-letter feature name @param {Object} sub - the substitution to add (an object like { sub: id or ids, by: id or ids }) @param {string} script='DFLT' @param {string} language='dflt'

Implementation

function Substitution(font) {
    Layout.call(this, font, 'gsub');
}