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- Given: input condition
`I`and output condition`O`.*Abstract*definition of a function ("what is to be done").- May be inconstructive (does not immediately yield an algorithm).
- Even if constructive, the corresponding algorithm may be too inefficient.

- Given: definition of a function
`f`.*Concrete*definition of a function ("how is it done").- Intended to yield (efficient) algorithm.

- Verification: show that
`f`implements corresponding specification.

*Definition of such a function and its verification needs more knowledge;
more knowledge gives better algorithms.*

Author: Wolfgang Schreiner

Last Modification: November 24, 1999