code: func interpolationFuncCeil() ast.Function {
  	return ast.Function{
  		ArgTypes:   []ast.Type{ast.TypeFloat},
  		ReturnType: ast.TypeInt,
  		Callback: func(args []interface{}) (interface{}, error) {
  			return int(math.Ceil(args[0].(float64))), nil
  		},
  	}
  }
docs: // interpolationFuncCeil returns the the least integer value greater than or equal to the argument