A five-level classification system for proteoform identifications by Smith, LM, Thomas, PM, Shortreed, MR, Schaffer, LV, Fellers, RT, Leduc, RD, Tucholski, T, Ge, Y, Agar, JN, Anderson, LC, Chamot-Rooke, J, Gault, J, Loo, JA, Paša-Tolić, L, Robinson, CV, Schlüter, H, Tsybin, YO, Vilaseca, M, Vizcaíno, JA, Danis, PO, Kelleher, NL