[Source:] pp 114-115, Symbolic Logic: A First Course (NOT 4 ed, 2011), by Gary Hardegree.
For want of brevity, I rewrite 'is on duty' as 'remains'.
1. the pool may NOT be used unless a lifeguard remains
2. Following the dictionary definition, this is equivalent to:
3. the pool may NOT be used except when a lifeguard remains
... = 4. the pool may NOT be used if a lifeguard is NOT on duty,
and the pool may be used if a lifeguard remains.
5. which, as noted earlier, is equivalent to the following biconditional,
6 . the pool may be used if and only if a lifeguard remains
comparing 6 with the original statement 1, we can discern ...
7. the strong sense of ‘unless’ is equivalent to ‘if and only if not’
comparing 6 with ... 1 mean 7? Please explain and show all steps and thought processes? This book appeared to have omitted some key steps.