Consider a functional of multiple functions
$A\left[y,z\right]=\int f\left(y\left(x\right),z\left(x\right)\right)dx$.
I would like to distinguish between partial and total functional derivatives, but I'm not sure if there is a standard notation for this. In other words, how can I distinguish between $z$ being independent of $y$, and $z$ being a functional of $y$?
For partial and total derivatives of functions, one typically uses a stylized d ($\partial$) and a roman (non-italic) d, respectively; therefore italic and roman deltas ($\delta$ and $\unicode[Times]{x3B4}$) seem like a natural choice for notating partial and total functional derivatives. (Unfortunately it seems that most fonts don't have a stylized delta analogous to $\partial$.)