6.1.5 Commas in FORMAT specifications
To support legacy codes, GNU Fortran allows the comma separator to be omitted immediately before and after character string edit descriptors in FORMAT statements.
PRINT 10, 2, 3
10 FORMAT ('FOO='I1' BAR='I2)
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https://gcc.gnu.org/onlinedocs/gcc-8.4.0/gfortran/Commas-in-FORMAT-specifications.html