[NTG-context] MetaFun labels and xsized

Dear list, I have this minimal MetaFun sample (the only one in my real document): \setupbodyfont[pagella] \starttext \startMPcode pair a ; a := (1, 1) ; pair b ; b := (1, 10) ; pair c ; c := (10, 10) ; pair d ; d := (10, 1) ; draw (a--b--c--d--a) xsized .75TextWidth; draw thelabel.lft("α",a) ; draw thelabel.lft("β",b) ; draw thelabel.top("γ",c) ; draw thelabel.rt("δ",d) ; \stopMPcode \stoptext Is there a way to relate adapt the labels to the xsized positions used? Many thanks for your help, Pablo -- http://www.ousia.tk