Declare a Namespace prefix.
This method declares a prefix in the current Namespace context; the prefix will remain in force until this context is popped, unless it is shadowed in a descendant context.
To declare a default Namespace, use the empty string. The prefix must not be "xml" or "xmlns".
Note that you must not declare a prefix after you've pushed and popped another Namespace.
Note that there is an asymmetry in this library: while {@link #getPrefix getPrefix} will not return the default "" prefix,even if you have declared one; to check for a default prefix, you have to look it up explicitly using {@link #getURI getURI}. This asymmetry exists to make it easier to look up prefixes for attribute names, where the default prefix is not allowed.
@param prefix The prefix to declare, or null for the emptystring.
@param uri The Namespace URI to associate with the prefix.
@return true if the prefix was legal, false otherwise
@see #processName
@see #getURI
@see #getPrefix