or / unless

Or and unless seem equivalent logical operators.

Sheaf morphism

A presheaf morphism's injectivity and surjectivity carries over to its induced homomorphism of stalks.
Injectivity of stalk homomorphism implies injectivity of morphism, provided both presheaves are sheaves.
To show surjectivity of morphism, one needs both injectivity and surjectivity of stalk homomorphism and the fact that both presheaves are sheaves.
Above, by injectivity and surjectivity of morphism I meant(perhaps incorrectly) the associated section maps are injective and surjective.