A variable of one type can be cast down to a more-specific target type by using a cast function. The name of the cast function is the name of the target type, followed by ?. The cast function returns an optional of the target type, with a value if the cast was successful, or no value if the cast was unsuccessful.

For A < B, with a:A, b:B, and c:A?:

b <- a;      // OK, as A < B
c <- A?(b);  // OK, but A < B so must use cast
The optional c may be used to check whether the cast succeeded, and if so, to retrieve the result:
if c? {
  f(c!);  // cast was successful, can do something with c
It is possible to cast an optional in the same way, without first checking for a value. The cast of an optional succeeds if that optional has a value, and that value can be cast to the target type. So for b:B? instead of b:B, the above example is the same.