Trace_context.Traceparent.of_value "xx":
  Error "trace context must be 55 bytes"
Trace_context.Traceparent.of_value "00":
  Error "trace context must be 55 bytes"
Trace_context.Traceparent.of_value "00-xxxx":
  Error "trace context must be 55 bytes"
Trace_context.Traceparent.of_value "00-xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx":
  Error "trace context must be 55 bytes"
Trace_context.Traceparent.of_value "00-0123456789abcdef0123456789abcdef":
  Error "trace context must be 55 bytes"
Trace_context.Traceparent.of_value "00-0123456789abcdef0123456789abcdef-xxxx":
  Error "trace context must be 55 bytes"
Trace_context.Traceparent.of_value "00-0123456789abcdef0123456789abcdef-xxxxxxxxxxxxxxxx":
  Error "trace context must be 55 bytes"
Trace_context.Traceparent.of_value "00-0123456789abcdef0123456789abcdef-0123456789abcdef":
  Error "trace context must be 55 bytes"
Trace_context.Traceparent.of_value "00-0123456789abcdef0123456789abcdef-0123456789abcdef-":
  Error "trace context must be 55 bytes"
Trace_context.Traceparent.of_value "00-0123456789abcdef0123456789abcdef-0123456789abcdef-00":
  Ok trace_id:"0123456789abcdef0123456789abcdef" parent_id:"0123456789abcdef"
Trace_context.Traceparent.of_value "00-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-01":
  Ok trace_id:"4bf92f3577b34da6a3ce929d0e0e4736" parent_id:"00f067aa0ba902b7"
Trace_context.Traceparent.of_value "03-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-01":
  Error "version is 3, expected 0"
Trace_context.Traceparent.of_value "00-ohnonohex7b34da6a3ce929d0e0e4736-00f067aa0ba902b7-01":
  Error "in trace id: invalid hex char: 'o'"
Trace_context.Traceparent.of_value "00-4bf92f3577b34da6a3ce929d0e0e4736-00f067aazzzzzzb7-01":
  Error "in span id: invalid hex char: 'z'"

Trace_context.Traceparent.to_value trace_id:"4bf92f3577b34da6a3ce929d0e0e4736" parent_id:"00f067aa0ba902b7":
  "00-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-00"

of_w3c_string "" -> Ok "" (len=0)
of_w3c_string "congo=t61rcwkgmze" -> Ok "congo=t61rcwkgmze" (len=17)
of_w3c_string "congo=t61rcwkgmze,rojo=00f067aa0ba902b7" -> Ok "congo=t61rcwkgmze,rojo=00f067aa0ba902b7" (len=39)
of_w3c_string "  vendor=value  " -> Ok "vendor=value" (len=12)
of_w3c_string "bad key=value" -> Ok "" (len=0)
of_w3c_string "key=val,=bad,other=fine" -> Ok "key=val,other=fine" (len=18)

set/replace: "vendor=new,other=xyz"
after delete: "vendor=new"
get vendor: "new"

Tracestate.name = "tracestate"
of_w3c_string round-trip: "a=b,c=d"
of_w3c_string empty: ""