Display BUFFER in an atomic window.
This function displays BUFFER in a new window that will be
combined with an existing window to form an atomic window. If
the existing window is already part of an atomic window, add the
new window to that atomic window. Operations like `split-window'
or `delete-window', when applied to a constituent of an atomic
window, are applied atomically to the root of that atomic window.
ALIST is an association list of symbols and values. The
following symbols can be used.
`window' specifies the existing window the new window shall be
combined with. Use `window-atom-root' to make the new window a
sibling of an atomic window's root. If an internal window is
specified here, all children of that window become part of the
atomic window too. If no window is specified, the new window
becomes a sibling of the selected window.
`side' denotes the side of the existing window where the new
window shall be located. Valid values are `below', `right',
`above' and `left'. The default is `below'. The return value is the new window, nil when creating that window failed.