in lib/builtin.js [498:510]
subString(ast, level) {
this.generator.emit('substr(');
this.getInstanceName(ast);
const args = ast.args;
this.generator.emit(', ');
const start = args[0];
this.generator.visitExpr(start, level);
this.generator.emit(', ');
const end = args[1];
this.generator.visitExpr(end, level);
this.generator.emit(')');
}