Unexpected output from simplify()

I have this code, looking for the derivative of this equation

syms mp dv vc c
expr2 = (1-((c^2)/(vc^2))*(exp(dv/c)-1))/(exp(dv/c))==mp;
simplify(diff(expr2,c))

I am expecting just a simple expression, but the output contains an extra item

c^2*dv - 2*c^3*exp(dv/c) + dv*vc^2 + 2*c^3 == 0 & vc ~= 0

What does the thing after the & sign mean? Why is it here?

Answer 1

[Askic V]

It is probably because you he a division by vc in your expression.

.../(vc^2)...

This means simplified expression is valid only if vc is different than zero.