The namespace problem arises when you want to build an XML document using pieces from one or more other documents created by others. When you do this, you can't very well rely on the others to avoid using the same names; for example, a document describing cooking supplies and a document describing inventory control might both the use tag <Stock> but mean very different things; and if you wanted to build an on-line store for cooks, this might be a real problem.
The reason the colon comes into play is that the leading candidate for a solution to the namespace problem relies on using colons; in the example above, you might want to distinguish inventories as <Retail:Stock> and soup-base as <Food:Stock>.
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