make-series does not allow variable reference for range

My understanding of the language that this is intentional. You cannot replace whole operators. You can this though:

let firstNum = 0;
let secondNum = 100;
let thirdNum = 20;
let T = datatable(x: int, group: string) [
 23, "red",
 44, "red",
 55, "red",
 66, "red",
 11, "green",
 43, "blue"
];
T | make-series avg(x) on x in range(firstNum,secondNum,thirdNum)  by group