Also, I'm trying to specify one of the GPIO as a (slow) clock, but it doesn't seem to like brackets. I need to escape it in TCL or it thinks the index is a command: set ::env(CLOCK_PORT) "foo\[126\]" but then OpenLane fails with this:

[INFO]: Storing configs into config.tcl ...
invalid command name "126"
    while executing
"126"
    ("uplevel" body line 1)
    invoked from within
"uplevel #0 ${cmd}"
    (procedure "set_log" line 3)
    invoked from within
"set_log ::env($index) $::env($index) $::env(GLB_CFG_FILE) 1"
    (procedure "prep" line 283)
    invoked from within
"prep {*}$args"
    (procedure "run_non_interactive_mode" line 12)
    invoked from within
"run_non_interactive_mode {*}$argv"
    invoked from within
"if { [info exists flags_map(-interactive)] || [info exists flags_map(-it)] } {
        puts_info "Running interactively"
        if { [info exists arg_values(-file)..."
    (file "/openLANE_flow/flow.tcl" line 223)
make[1]: *** [Makefile:43: user_project_wrapper] Error 1
make[1]: Leaving directory '/home/mrg/openram_testchip/openlane'
make: *** [Makefile:71: user_project_wrapper] Error 2