Fig 1.

(a) Metabolite A and its MFP ${\mathrm{A}}_{\left\{\mathrm{2,3}\right\}}^{\left\{\mathrm{2,3},4\right\}}$ with parent fragment N = {2,3,4} and product fragment K = {2,3}. (b) The tandemer distribution matrix ${\left[\mathrm{A}\right]}_{\left\{\mathrm{2,3}\right\}}^{\left\{\mathrm{2,3},4\right\}}$. The abundance of infeasible tandemers in ${\left[\mathrm{A}\right]}_{\left\{\mathrm{2,3}\right\}}^{\left\{\mathrm{2,3},4\right\}}$ is, by definition, zero.