old_contents: export default function(signal, scope) {
    var op = scope.addSignal(signal.name, signal.value);
    if (signal.react === false) op.react = false;
    if (signal.bind) scope.addBinding(signal.name, signal.bind);
  }
  
new_contents: import {error} from 'vega-util';
  
  export default function(signal, scope) {
    var name = signal.name;
  
    if (signal.outer) {
      // signal must already be defined, raise error if not
      if (!scope.signals[name]) {
        error('Missing prior signal definition for "outer": ' + name);
      }
    } else {
      // define a new signal in the current scope
      var op = scope.addSignal(name, signal.value);
      if (signal.react === false) op.react = false;
      if (signal.bind) scope.addBinding(name, signal.bind);
    }
  }
  
subject: Add support for signal updates to outer scope.