Produce \header in function

[Павел]

Hello lilyponders! I want to produce \header block in my function, to 
use my function instead \header block. But I can't seem to find the 
correct syntax for this. It doesn't work like that:

genheader = #(define-music-function
    (mystring) ( string? ) #{
        \header { piece = \markup\bold #mystring }
    #})