Tuple composition does not merge?

[blais]

Hi!
I was expecting the semantics of tuple composition to merge and accrue (for lists) the contents.
For example:

a = {                                  
  config = {                           
    exponent = { constant = 0.30 }     
  }                                    
};                                     
                                       
b = {                                  
  config = {                           
    base = { constant = 2.0 }          
  }                                    
};                                     
                                       
default = a b                                                                 

but the result is

+- default
|  +- config
|  |  +- base
|  |  |  +- constant => 2.0

I was expecting both sets of attributes in default.
Same behavior with compose_all().

Is tuple merging possible in this implementation of gcl, or is only applied in literal syntax?
Thank you,