Expression2 Help Request: Rotating an entity on one axis

Depending on the server's setup you can put the E2 directly on it and do the following:

@persist Prop:entity
if (first())
{
    # lines starting with # are comments in E2
    Prop = entity():isWeldedTo()
    Speed = 10 #10 units every 100ms
}
interval(100) #how long to wait until we execute again, 
# could use runOnTick(1) for every server frame

#this sets Prop (in this case welded to the E2, you can have this instead:
#@input Prop:entity
Prop:setAng(ang(Pitch, Yaw, Roll))

#here's where you change what you want to rotate on:
#you can put Pitch, Yaw, or Roll

#remember: pitch is up and down, yaw left and right, and roll is top to left / top to 
#          right
Yaw += Speed