MErge filename
Gets an existing circuit file, without clearing memory.
The first comment line of the file being read is the new title, and replaces the existing title.
Comments in the circuit file are stored, unless they start with *+ in which case they are thrown away.
`Dot cards' are set up, but not executed. This means that variables and options are changed, but simulation commands are not actually done. As an example, the options command is actually performed, since it only sets up variables. The ac command is not performed, but its parameters are stored, so that a plain ac command will perform the analysis specified in the file.
Any circuit already in memory is kept. New elements with duplicate labels replace the old ones. New elements that are not duplicates are added to the end of the list, as if the files were appended.