Raise NotImplementedError when mixing Order at 0 and oo.
Fix Order(exp(x), x, oo) (issue 4108).
Fix evalf for zero sum (issue 3273)
Merge branch 'master' into series
Merge pull request #2618 from skirpichev/3273-evalf-0sum

Fix evalf for zero sum (issue 3273)
Remove parentheses on raise.
Merge pull request #2615 from jrioux/series

Fixing issues with Order at Infinity.
Implement a new syntax for multivariable Order symbol

    Supported input:
    O(expr)
    O(expr, (var1, pt1), (var2, pt2), ...)
    O(expr, var1, var2, ...) == O(expr, (var1, 0), (var2, 0), ...)
Fix Order syntax in some tests
Fix tests for as_expr_variables method
Fix printing tests
Test that Order(x, 2 - x) raises now TypeError, not NotImplementedError
Test that variables are unique symbols
Test that O(O(x), (x, oo)) raises an exception
O(O(expr, vp)) -> O(expr, vp)
Add Order.variables to the return value of Order.free_symbols