syntax for very simple derivative function

Readers,

Two variables 'M', 'T', each contain a column of numbers.

Have tried to create a derivative function to see how 'M' varies with
respect to 'T':

dm<-D(m)(dm/dt)~dt
dm(m=M,t=T)

but R returns an error:

Error: could not find function "dm"

What is my mistake please?

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R2151