a field-theory motivated approach to computer algebra

## Coordinate

Declare a symbol to be a coordinate label.
Declare a symbol to be a coordinate label (useful in combination with Depends). This is required if you want to write a derivative with respect to a coordinate: if you write
\partial{#}::PartialDerivative. ex:= A(x,y) + \partial{B(x,y)}_{x};
$$\displaystyle{}A((x, y))+\partial(B((x, y)))\,_{x}$$
will by default be seen as incorrect because the $x$ in the second term will be considered an index label, not a coordinate. The input
{ x, y }::Coordinate. ex:=A(x,y) + \partial{B(x,y)}_{x};
$$\displaystyle{}A((x, y))+\partial(B((x, y)))\,_{x}$$
is allowed and interpreted in the right way.