SUB read_data(x(),y(),ndata)
    ! read file for position of particle at regular intervals 
    OPEN #1: name "xy.dat",access input
    LET t = 0
    DO while more #1
       LET t = t + 1
       INPUT #1: x(t),y(t)
    LOOP
    CLOSE #1
    LET ndata = t                 ! # of data points
END SUB