Defining custom global constants
Is it possible to define a custom constant like pi? One that is handled symbolically, one that can be used without prior definition...
For example, could it be possible to define "tau" to equal 2*pi for my Sage installation so that when I open my notebook, I can call "tau" without needing to define it in every cell?
And to take it a step further, would it be possible to replace constants? Like pi with tau/2? This isn't as important, but it would be very nice!