docs: function(parent) {
      function error(usage) {
        return new Error('Template theme fields are not permitted '+ usage +':\n>>> '+ Node.toString(parent));
      }
  
      // Check for arguments implementation:
      if (parent.type === NodeType.ARGUMENTS) {
        throw error('as arguments');
      }
  
      // Check for interpolations implementation:
      if (parent.type === NodeType.INTERPOLATION) {
        throw error('in interpolations');
      }
  
      // Check for operations implementation:
      for (var i=0; i < parent.content.length; i++) {
        if (NodeType.OPERATOR.test(parent.content[i].type)) {
          throw error('in operations');
        }
      }
    }