This is probably not what you are expecting, and it is indeed a simplistic solution

. But, this is how I do it, and I hope it works (at least transiently) for you.
The Constraints tab for use cases has a pair of buttons with "pointing finger" icons. Click on your first "Proposed" precondition, then use the button pointing "up" to put it at the begining of the list. Proceed to group all your "Proposed" preconditions together, your "Mandatory" ones after these, and so on... Then gorup your post conditions, and whatever other constraints you have.
What I obtain in the RTF is very clear for non-technical users, and should not confuse them at all. The following example is from a community census (this is a real system, for a municipality in the state of Oaxaca):
Take census
Alias: Interview familiy and fill forms
Type: Use case
Status: Approved...
The person in charge of taking the cesus visits a family, at the family's home, and interviews individual members in order to fill in the information required in the forms.
Constraints
. Mandatory Pre-condition . Train persons in charge of taking census.
. Mandatory Pre-condition . Have the required pre-printed forms.
. Mandatory Post-condition . Check that number ("folio") in individual forms correspond to the family's consecutive ("folio") number.
I hope this helps.
Jaime Gonzalez