Define command with operator in string

Dear Fluka,
I am using a preprocessor command in my input data to define the particle type.
For instance, I use
#define Particle NEUTRON
and my beam card is
BEAM -99999.0 0 0 0 -1.0$Particle
which corresponds to neutron and works well.

However, if the particle is a PION+
#define Particle PION+
The input data does not work because it seems fluka/flair tries to perform an addition here.
The error is

*** Invalid operator at the beginning/end of a subexpression, execution terminated ***
 Abort called from EXPREV reason INVALID OPERATOR IN SUB-EXPR! Run stopped!

Is there a way to avoid this kind of problem?

Dear @thomas.f
thank you for the bug report. Indeed this is a shortcoming in the fluka expression handling. FLUKA is detecting the operator in the string and thinks that this is an expression and it fails.
As a temporary solution to your problem would be to use multiple BEAM cards one for each particle surrounded by #if..#elif..#endif cards