Some REXX help

[Huegel, Thomas]

I copied this line from the REXX VM/REFERENCE

It is from a sample SOCKETS SERVER exec (page 16.2).
When I try to execute it I get errors.
I have not seen code like this before, I wonder if anyone can explain the 
technique to me?

if rest&notsym;='' then call error 'E', 24, 'Invalid parameters specified'

Its the rest&notsym;=''    that I don't understand.


Thanks