code: func interpolationFuncAbs() ast.Function {
  	return ast.Function{
  		ArgTypes:   []ast.Type{ast.TypeFloat},
  		ReturnType: ast.TypeFloat,
  		Callback: func(args []interface{}) (interface{}, error) {
  			return math.Abs(args[0].(float64)), nil
  		},
  	}
  }
docs: // interpolationFuncAbs returns the absolute value of a given float.