extracting all elem names of an attribute

Hello,

How best is the following text extracted
from the following xml, possibly using keys
or some other xslt constructs?

====
size: small, medium, large
crust: thin, pan
topping: olives, pepperoni, peppers
====

<db>
   <large category="size">
      <thin category="crust">
         <olives category="topping/>
         <pepperoni category="topping/>
      </thin>
      <pan category="crust">
         <pepperoni category="topping/>
      </pan>
   </large>
   <medium category="size">
      <pan category="crust">
         <pepperoni category="topping/>
         <olives category="topping/>
         <peppers category="topping/>
      </pan>
   </medium>
   <small category="size">
      <thin category="crust">
         <olives category="topping/>
         <pepperoni category="topping/>
         <peepers category="topping/>
      </thin>
      <pan category="crust">
         <pepperoni category="topping/>
         <olives category="topping/>
         <peepers category="topping/>
      </pan>
   </small>
</db>

The basic approach involves:
  
   - using keys to get a unique list of possible values for the @category attribute
   - iterating over these values 
        - printing each followed by a colon
        - getting a unique list of all xml element names which contain this particular attribute value
             - iterating over these values
                  printing each
                  printing an endline

Many thanks and Best regards,
Saverio

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