A keyword placed after the "defined" list in 'set contourfill' is rejected.
For example,
set contourfill defined [-2:0] "blue", [0:2] "red" firstlinetype 5
shows the error message "Unrecognized option".
Probably related: inside a braced block
if (1) {
set contourfill defined [-2:2] "blue"
print "foo"
}
also shows same error message.
gnuplot 6.1 (last modified 2026-07-28), built from source.
macOS 15.6.1 (arm64).
The first one is a misleading error message ('defined' and 'firstlinetype' are mutually exclusive options).
The second one is a bug.
Fixed in 6.1, pending for 6.0