 The connection between combinators and types may be explained on the example of the identity combinator. Compound combinatory terms are formed by the application operation. Premises of modus ponens can be joined by fusion (denoted by \(\circ\)), which is like the application operation in the strongest relevance logic \(B\). \(\textsf{I}x \triangleright x\) and so if \(x\)'s type is \(A\), then \(\textsf{I}x\)'s type should imply \(A\).  Furthermore, \(\textsf{I}x\)'s type should be of the form \(X\circ A\), for some type \(X\); then \(\textsf{I}\) can be of type \(A\rightarrow A\). In the example, we fixed \(x\)'s type, however, \(\textsf{I}\) can be applied to any term, hence, it is more accurate to say that \(A\rightarrow A\) is the type schema of \(\textsf{I}\), or that \(\textsf{I}\)'s type can be any formula of the form of self-implication.