6.1.33 Legacy PARAMETER statements

For compatibility, GNU Fortran supports legacy PARAMETER statements without parentheses with -std=legacy. A warning is emitted if used with -std=gnu, and an error is acknowledged with a real Fortran standard flag (-std=f95, etc...). These statements take the following form:

implicit real (E)
parameter e = 2.718282
real c
parameter c = 3.0e8

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https://gcc.gnu.org/onlinedocs/gcc-10.2.0/gfortran/Legacy-PARAMETER-statements.html