*Let X and X' be two uniform spaces. For each uniformly continuous mapping f:X→X′ there is a unique uniformly continuous mapping f^:X^→X^′ such that the diagram
Xi↓X^i′f^X′↓i′X^′
is commutative (*), where i:X→X^ and i′:X′→X^′ are the canonical mappings.
Apply Theorem 3 to the function i′∘f:X→X^′.
Bourbaki puts the proof directly after the statement, so what is above is the statement and its proof together.