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**(1)**- In this chapter, we focus on the
*language*aspect of logic; the*reasoning*aspect is investigated in Appendix*Proving*. A third aspect, the automatization of problem solving on computers, is discussed in another course "Logic for Computer Science". **(2)**- Logical constants can be considered as connectives of arity 0, i.e., as connectives that do not take any arguments.
**(3)**- See the followup comments on the definition of "union" and "powerset".
**(4)**- This does not contradict the statement that for every positive
`n`in**N**, there is a unique pair -`n`in**Z**and +`n`in**Z**. Rather it shows us that the union of two disjoint copies of an infinite set`S`still has the same size as`S`, i.e., that our intuition of set sizes cannot be automatically applied to the infinite case. **(5)**- In computer science, trees grow downwards! ;-)
**(6)**- The corresponding propositions may be nevertheless true.

Author: Wolfgang Schreiner

Last Modification: October 4, 1999