Graph title string:
    GTPARAMS	Database file name
    GTTITLE	Selection criteria
    GTSUBTITLE	Marker definitions
    GTCOMMENTS	Sizer parameter

Histogram title string:
    GTPARAMS	Database file name
    GTTITLE	Selection criteria
    GTSUBTITLE	Selection criteria for overplotted histograms
    GTCOMMENTS	Histogram parameters (z1, z2, nbins, top-closed, nindef)

When adding a new function or arithmetic operator, changes must be made to:
    gettoken.x
    parse.x	specifically to procedure pop_op
    evaluate.gx
