XPinsAttr()
  Insert Attributes in a string
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   Function:   XPinsAttr()

               This is an assembly written function that inserts
               attributes in a string. Used by XPdisplay(), for instance,
               and some other functions.

   Syntax:     XPinsAttr(<cString>,<nAttr1>,<nAttr2>) --> cScreen

   Arguments:  <cString> is a string. <nAttr1> and <nAttr2> are the
               attributes to insert in the string. <nAttr1> is the attribute
               used by default, when a '~' is encountered, it is toggled
               to <nAttr2>. a '~' toggles back again. This makes it possible
               to make strings with changing attributes.

   Returns:    a Buffer that can be used by RestScreen().

   Usage: