Notion of not specific to compilers

• and need not be used in compilers

A generalization of idea of replacing strings with symbols

• ensure we can use pointer equality instead of structural equality
• avoid constructing duplicate objects that are equal
• only works for immutable trees

Trivially, if pointers equal, then so is structure

Ensuring converse (if structure equal, so are pointers):

• replace constructor with 'hashing constructor'
• constructor maps compound object to its components
• with hash consing, we maintain a mapping from components to compound object
• if such object already created, we return same address

• Object Equality Profiling