sheaf -- make a coherent sheaf

Ways to use sheaf :

• sheaf(Module) -- make a coherent sheaf
• sheaf(Ring) -- make a coherent sheaf of rings
• sheaf(Variety) -- make a coherent sheaf
• sheaf(Variety,Module) -- make a coherent sheaf
• sheaf(Variety,Ring) -- make a coherent sheaf of rings

For the programmer

The object sheaf is a method function.