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$$
{\frac  {{\textbf  {H}}\;\;\vdash \;\;{WD}({\textbf  {Q}}(f(x)\mapsto g(x)))\qquad {\textbf  {H}}\;\;\vdash \;\;{\textbf  {Q}}(g(f(x)))}{{\textbf  {H}}\;\;\vdash \;\;{\textbf  {Q}}((f\fcomp g)(x))}}
$$
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