Solved
How to define a function with constants
I'm trying to define a function where a,b and R are constants. I set the labels to constant but I get an error. 
The error message stems from Primes numeric engine as it can't do calculation with variables not assigned a value.
As you have seen you still can do symbolic operations without any problem so it does no harm but just is bad looking.
To avoid the error you have two options:
1) Define the function as P(v,T,a,b):=.... or as P(a,b,v,T):=...
2) assign a and b values before defining. The type clear.sym(a,b) and then do your symbolic calculations
To clear the variables after that completely also for numeric use, you may type clear(a,b)
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