[HACKERS] Undefined psql variables

I would like an option where either psql can provide an alternate value when an undefined variable is referenced, or a way to detect that a specific variable is undefined and replace it with a defined variable.

\set_if_undefined var_name 'default_value'

# select :'x' as value_of_x;

ERROR:  syntax error at or near ":"

LINE 1: select :'x' as value_of_x;

               ^

# \pset variable_default ''

analytics=# select :'x' as value_of_x;

 value_of_x

------------

(1 row)

# \pset variable_default ''

# select :'x' as value_of_x;

ERROR:  syntax error at or near ":"

LINE 1: select :'x' as value_of_x;

               ^

\pset variable_default 'default1'

select :'required_var1' as required_var1 \gset

\pset variable_default 'default2'

select :'required_var2' as required_var2 \gset

-- reset to default behavior

\pset variable_default error