How to use variable as params in less function, and execute if statement?

How to use variable as params in less function, and execute if statement?

@a: 'a';
@b: 'b';
.expand(@a, @b);


.expand(@a, @b) {
  // @if @a == 'a' && @b == 'b' {

  // } @else if @a == 'a1' && @b == 'b1' {

  // }
}