[FOM] A textbook on logic with natural deduction

[Andrej Bauer]

Dear FOMers,

I am teaching a freshman course in logic and set theory. The formalism
is kept very low, indeed, I never really explain the difference
between syntax and semantics, I am just trying to teach the students
what a (real-life) proof is. However, we did learn natural-deduction
style proofs as trees. There are several foreign students who asked me
for an English textbook on the topic. They seem very happy with "Naive
set theory" by Halmos, but I couldn't really find an _introductory_
textbook that would cover logic in natural deduction style. Can
someone recomment a good textbook or available lecture notes that are
written in natural-deduction style (trees, not proof boxes, I know I
should have used boxes... but it is too late for this year)? This is
for a freshman course.

Thank you,

Andrej Bauer