def filtra_capcalera(nfcorreu, camps, nffiltrat):
    trobat = False
    with open(nfcorreu, 'r') as f, open(nffiltrat, 'w') as g:
        for linia in f:
            trobat = linia == '\n'
            if trobat:
                break
            l = linia.split(': ')
            if len(l) > 1:
                camp = l[0]
            if camp in camps:
                g.write(linia)