I have a question about page 9 in Proofs and Types.The given $\rightarrow$Introduction rule says that $A \rightarrow B$ can be deduced from $B$ if the deduction of $B$ contains an arbitrary number of $A$s (0 or more).  How can this be true if there are zero $A$s in the deduction of $B$?
Discharged hypotheses in ->Introduction
lo.logic;natural deduction