Where there is a choice of several acceptable forms, that form is selected which......
Where it is important to distinguish different norms on E, we shall use the notation......
Where it is possible, we outline the proofs so that the reader will not have to hunt for obscure references.
Where we could, we have chosen these examples from naturally occurring mathematical structures. [Note the double r in occurring.]
There are many situations where this occurs naturally.
This is where the notion of an upper gradient comes in.
Important cases are where S=......
[Note that where may sometimes be ambiguous: “where i∊ I'' after a formula can mean either “for all i∊ I'' or “for some i∊ I''.]