src/semantic.js
function isRIdent(node) {
    return node &&
           node.type &&
           node.type === T.IDENT &&
           node.content === 'R'
}
function isRIdent(node) {
    return node &&
           node.type &&
           node.type === T.IDENT &&
           node.content === 'R'
}
function isRIdent ( node ) { return node && node . type && node . type === T . IDENT && node . content === 'R' }
whether a node is and identifier and its content is `R`
whether a node is and identifier and its content is R