[NTG-context] Metapost question

Hello, does someone know how, if at all, one can printout some variable? Like Emean:=(2*Eone+Ezero+2*Etwo)/5.; label.urt(btex $A = x$ .... where x is the value of Emean? I frequently create some diagrams, modify them and then forget to change the label. (If one could set the number of digits, it would be even nicer.) Tobias