Defining @ constraint with IntervalConstraintProgramming

General Usage
1

I am trying to define a constraint using the IntervalConstraintProgramming.jl package as follows:

a=3
C = @constraint x^2 + y^2 - $a <= 0

Here, I wanted x and y to be treated as variables and a as a constant, defined elsewhere. But I get the following error:

UndefVarError: `a` not defined

Stacktrace:
 [1] top-level scope
   @ C:\Users\user\.julia\packages\IntervalConstraintProgramming\8OYsb\src\separator.jl:156

If I remove the ‘$’ before a, I get:

Separator:
  - variables: x, y, a
  - expression: (x ^ 2 + y ^ 2) - a ∈ [-∞, 0]

But here, a is also treated as a variable, which is not what I wanted. Any help would be deeply appreciated. Thanks in advance!