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'
}
whether a node is and identifier and its content is `R`