program     := declaration* term
declaration := func '=' term ';'
term        := appterm term
             | appterm
appterm     := func
             | ident
             | '\' ident+ '.' term
             | '(' term ')'
func        := 'A' | 'B' | ... | 'Z'
ident       := 'a' | 'b' | ... | 'z'