check Subroutine

    subroutine check(condition, msg, code, warn)
        !! version: experimental
        !!
        !! Checks the value of a logical condition
        !! ([Specification](../page/specs/stdlib_error.html#description))
        !!
        !!##### Behavior
        !!
        !! If `condition == .false.` and:
        !!
        !!   * No other arguments are provided, it stops the program with the default
        !!     message and exit code `1`;
        !!   * `msg` is provided, it prints the value of `msg`;
        !!   * `code` is provided, it stops the program with the given exit code;
        !!   * `warn` is provided and `.true.`, it doesn't stop the program and prints
        !!     the message.
        !!
        !!##### Examples
        !!
        !!* If `a /= 5`, stops the program with exit code `1`
        !!  and prints `Check failed.`
        !!``` fortran
        !!  call check(a == 5)
        !!```
        !!
        !!* As above, but prints `a == 5 failed`.
        !!``` fortran
        !!  call check(a == 5, msg='a == 5 failed.')
        !!```
        !!
        !!* As above, but doesn't stop the program.
        !!``` fortran
        !!  call check(a == 5, msg='a == 5 failed.', warn=.true.)
        !!```
        !!
        !!* As example #2, but stops the program with exit code `77`
        !!``` fortran
        !!  call check(a == 5, msg='a == 5 failed.', code=77)
        !!```

        !
        ! Arguments
        ! ---------

        logical, intent(in) :: condition
        character(*), intent(in), optional :: msg
        integer, intent(in), optional :: code
        logical, intent(in), optional :: warn
        character(*), parameter :: msg_default = 'Check failed.'

        if (.not. condition) then
            if (optval(warn, .false.)) then
                write(stderr,*) optval(msg, msg_default)
            else
                call error_stop(optval(msg, msg_default), optval(code, 1))
            end if
        end if

    end subroutine check