• Subroutines may not be well documented

• User must understand documentation

• Many subroutines: takes time to become familiar with the collection

• User may rewrite subroutine rather than reusing it

• User might make mistakes, e.g. wrong type of units of argument

Constructive proof: given a theorem &forall x &exist y P(x,y) , prove it by constructing a y that satisfies the theorem.

specification &rarr theorem &rarr proof &rarr program

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