Panasonic FP Series Programming Manual page 258

Body In channel_select you find the integer value (0, 1...n) for the selection of channel_0 or
channel_1. The result will be written into output.
LD
ST When programming with structured text, enter the following:
output:=MUX( K:=
IN1:= channel_1
channel_select , IN0:=
);
Selection instructions
channel_0 ,
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