The Global Campaign against Headache, aged 21: a critical self-appraisal, and plans for phase 2 - Timothy J. Steiner; Raquel Gil-Gouveia; Andreas K. Husøy; Rigmor Jensen; Zaza Katsarava; Paolo Martelletti; Lars Jacob Stovner; Derya Uluduz (The Journal of Headache and Pain)