We have formalized the BCI-algebras closely following the book  pp. 16-19 and pp. 58-65. Firstly, the article focuses on the properties of the element and then the definition and properties of congruences and quotient algebras are given. Quotient algebras are the basic tools for exploring the structures of BCI-algebras.
 Grzegorz Bancerek. Binary operations. Formalized Mathematics, 1(1):175-180, 1990.
 Czesław Byliński. Functions and their basic properties. Formalized Mathematics, 1(1):55-65, 1990.
 Czesław Byliński. Functions from a set to a set. Formalized Mathematics, 1(1):153-164, 1990.