docs: function createOperatorNode(operand, nodes = []) {
    return (scope, opts) => {
      const args = nodes.map(node => node(scope, opts));
  
      if (typeof operand === 'function') {
        return operand(...args);
      }
  
      const operator = get(opts.functions, operand);
  
      if (!operator) {
        throw new ReferenceError(`Function '${operand}' isn't declared`);
      }
  
      return operator(...args);
    };
  }