the calling procedure does exist , the procedure that is being called may not exist.
it could be a customer specific adapted stored procedure.
some customers have them , some haven.t . that's why this is necessary to create code to automatically redirect the code to this client specific stored proc.
{
proc spd_common

begin
if dba-object 'spc_common' then
exec 'spc_common';
--client spec section
else
--common section