4.32.1 Writef
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Availability::- use_module(library(writef)).(can be autoloaded)
Source[deprecated]swritef(-String, +Format, +Arguments)
Equivalent to writef/2, but “writes” the result on String instead of the current output stream. Example: 
?- swritef(S, '%15L%w', ['Hello', 'World']).

S = "Hello          World"