Force evaluation : unquote

unquote is used to evaluate inside a quoted expression.
For example in an affectation, the variable is automatically quoted (not evaluated) so that the user does not have to quote it explicitely each time he want to modify it's value. In some circumstances, you might however want to evaluate it.
Input:

purge(b);a:=b;unquote(a):=3

Output :

b contains 3, hence a evals to 3