src/vs/workbench/contrib/tasks/browser/terminalTaskSystem.ts/TerminalTaskSystem/_findUnresolvedVariables
class TerminalTaskSystem:	private _findUnresolvedVariables(variables: Set<string>, alreadyResolved: Map<string, string>): Set<string> {
		if (alreadyResolved.size === 0) {
			return variables;
		}
		const unresolved = new Set<string>();
		for (const variable of variables) {
			if (!alreadyResolved.has(variable.substring(2, variable.length - 1))) {
				unresolved.add(variable);
			}
		}
		return unresolved;
	}