To read in expressions from a file, one can do
ReadList[file, Expression, 3]. How to do the opposite, i.e. remove a number of expressions from a file, modifying the file on the disk?
In a Sequential file you can´t delete entries. The only option is shifting the "tail" of the file upwards.
A filtered stream reading function:
Define a stream to read.
Create a reader on that stream.
Read from the filtered stream reader.