code: func interpolationFuncContains() ast.Function {
  	return ast.Function{
  		ArgTypes:   []ast.Type{ast.TypeList, ast.TypeString},
  		ReturnType: ast.TypeBool,
  		Callback: func(args []interface{}) (interface{}, error) {
  			_, err := interpolationFuncIndex().Callback(args)
  			if err != nil {
  				return false, nil
  			}
  			return true, nil
  		},
  	}
  }
docs: // interpolationFuncContains returns true if an element is in the list
  // and return false otherwise