How can a mutable value be constant?

[ctjlewis]

Very interested to hear the logic behind this.

[mbostock]

Maybe start here? Introduction to Mutable State / Observable / Observable

You need to assign to a mutable using the mutable operator; mutable foo and foo are two different identifiers.

{
  mutable foo = 5;
}

(Also, please be courteous. We’re happy to help but the tone of your request—or at least the image—is likely to discourage people from responding!)