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These functions deal with storing and retrieving data from the
per-window data area which you can allocate for a single window or a
class of windows. The size of the area can be specified on a per-class
basis as part of the WNDCLASS structure used in the RegisterClass()
function. If it is done this way, then whenever a window of that
class is created, the window extra bytes area is automatically
allocated. Alternatively, you can allocate a window extra bytes area
on a per-window basis using the SetWindowExtra() function. The window
extra byte area is automatically freed by MEWEL when a window is
destroyed.

 GetWindowByte
 GetWindowExtra
 GetWindowLong
 GetWindowPtr
 GetWindowWord
 SetWindowByte
 SetWindowExtra
 SetWindowLong
 SetWindowPtr
 SetWindowWord