Immutable maps.  Keys are ordered by construction function specified
 when creating empty maps or by F# structural comparison if no
 construction function is specified.

  
   Maps based on structural comparison are  
   efficient for small keys. They are not a suitable choice if keys are recursive data structures 
   or require non-structural comparison semantics.
 
 Immutable maps.  A constraint tag carries information about the class of key-comparers being used.