# How to plot in Sage only part of f(x,y) bound by inequalities?

I want to use SageMath to plot in 3D a function of two variables z=f(x,y) that is bound by a number of inequalities. As a simple example, say:

f(x,y) = x+2y

This is just a tilted plane; doesn't look too exciting. But now... Restrict it to only points that satisfy both of these inequalities:

y<x (so, boundary is line with slope 1)

y>-x (so, boundary is line with slope -1)

Note that these boundaries intersect at (x,y)=(0,0) I would like a 3D rotatable view of a plane with the edges at the boundary lines, and a corner at the origin.

Later I would like to add more inequalities, maybe a few other bondaries so that it is limited to a triangular or other polygonal region, or perhaps $$x^2 + y^2 < 1$$ so it would be a tilted stretch quarter-circle.

How do I set these boundaries? When I looked up plotting bondaries, my web search only found how to set min,max for (x,y) for the purposes of displaying the plot, and not how to confine the region itself.