Expressionse,b,f,a∷=xVariable∣x⇒bAbstraction∣f←aApplicationVariablesx∈k,x,y,…Numbersn∈0,1,2,… \small\begin{array}{lrcll} \mathrm{Expressions}\quad & e,b,f,a & {\Coloneqq} & x & \quad\mathrm{Variable} \\ & & {\mid} & x\Rightarrow b & \quad\mathrm{Abstraction} \\ & & {\mid} & f\leftarrow a & \quad\mathrm{Application} \\ \mathrm{Variables} & x ...