Circle procedure - uses the arc command which requires variables x (angle) and r (radius):  

lt 45 arc :x :r

Concentric circles procedure - uses the circle procedure.  The version I used for the graphic had 50 circles, with an initial radius of 3 increasing by 3 each time.  A variable 'amount' is defined in the procedure for the radius using the 'make' command:

make "amount 3 repeat 50[circle 360 :amount make "amount :amount + 3]

General concentric circles - this can be generalised for any number of circles, n, any initial radius, r, and any increment, d:

make "amount :r repeat :n[circle 360 :amount make "amount :amount + :d]