Traditionally, DEC did not provide an I-beam cursor, only a solid block or a lower plan, through a sequence of DECSCUSR ( CSI Sp q ):
CSI 1 Sp q == blinking block CSI 2 Sp q == solid block CSI 3 Sp q == blinking underbar CSI 4 Sp q == solid underbar
Recently in xterm we expanded this enumeration to define
CSI 5 Sp q == blinking vertical bar CSI 6 Sp q == solid vertical bar
Perhaps urxvt might suggest a similar coding scheme.
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