possible request for new function to elliptic curves section

[American Citizen <website.reader3@gmail.com>]

Would it be possible to have a new function added to GP Pari, such that 
supplying just the x-coordinate for an elliptic curve would return a 0 
or a 1 depending upon whether or not x is a legitimate value? This is 
very similar to ellisoncurve(E,p) but p = [x,y] and both x,y must be 
known. I am asking just for something like

ellxok(E,x) = 0,1 depending upon whether or not x is a legitimate value 
creating a rational y value

or ellxonE(E,x) or something like this?

Randall