Programmings›Lean›Chapter 2. Propositions and Proofs2.9 Rewriting with EqualityRewriting is the main way to use equality in Lean.
Programmings›Lean›Chapter 2. Propositions and Proofs2.8 Equality BasicsEquality expresses that two terms are identical.
Maths›00. General Mathematics1.3 Equality, Identity, and EquivalenceDifferent notions of sameness in mathematics: strict equality, structural identity, and equivalence relations.