<math display="block"> <mmlrow> <mmlo>&#x2202;</mo><mmlsub> <mmlover accent='true'> <mmli>m</mi> <mmlo>&#x02D9;</mo> </mover> <mmli>s</mi> </msub> <mmlo>=</mo><mmlsub> <mmlover accent='true'> <mmli>m</mi> <mmlo>&#x02D9;</mo> </mover> <mmli>a</mi> </msub> <mmlo>&#x2202;</mo><mmlsub> <mmli>w</mi> <mmli>a</mi> </msub> </mrow> </math>