Compound expressions in certain function arguments, and elsewhere?

[davidfarmer]

Some functions are commonly written without parentheses around their
arguments in certain cases, such as

cos 2pi = 1

A convention which seems workable is to consider the argument to continue
until white space or the end of the expression.

The some convention would interpret
e^2x as exp(2x)

Another situation where a similar convention might be workable is in limits
of integrals as in

int_0^2pi cos x dx = 0 .

Except that it would be weird to allow that in the lower limit but not the upper
limit:

int_18pi^26pi sin x = 0

There it is the ^ and not a space that signals the end of the lower limit.

But if that works, then why not

sum_n=1^infty n^-2 = pi/6 ?

I am not advocating either way. I find
sum_n=1^infty harder to read than sum_(n=1)^infty
so maybe there is nothing to be gained by allowing that flexibility everywhere.

It would be nice to have a sharp line delimiting when such grouping has
parentheses optional. Maybe this will become clearer as we test more examples.