    # Display a CA trace for selected residues by adding a
    # distance between CA atoms of subsequent residues.

    # store current selection
    DefPropAtom 'prev_sel' 'selected'
    DefPropDist 'prev_sel' 'selected'

    # deselect all distances so that we can set the color only
    # on the new distances and set the selection to the previously
    # selected + new distances at the end.
    SelectDist '0'

    # select CA atoms of selected residues
    SelectAtom 'res.selected & ca'

    # add distances for all pairs of CA that are less than
    # 5 A apart
    CalcDist 5.0 1 1 0 0 0 1

    # set property for new distances
    DefPropDist 'new_dist' 'selected'

    # make new distances red, and set their line width to 5
    ColorDist 1 0 0
    LineDist solid 5

    # select the new distances that are not between
    # subsequent residues, and remove them
    SelectDist 'new_dist & res2.num > res1.num + 1'
    RemoveDist

    # restore previous selection, with new distances added
    SelectAtom 'prev_sel'
    SelectDist 'new_dist | prev_sel'