Solution
= Solution
The second bin is forced to contain $A,B$, leaving the other four letters in the first bin. Since order within a bin is irrelevant, this gives exactly
$$
\boxed{1}
$$
<set partition> of the specified bin sizes.
= Solution
The second bin is forced to contain $A,B$, leaving the other four letters in the first bin. Since order within a bin is irrelevant, this gives exactly
$$
\boxed{1}
$$
<set partition> of the specified bin sizes.