The use of search history can help create an elaborate search strategy. 

Each time you run a search, a search step is automatically created in the search history section.

Here search step number 1 corresponds to the query shown in the previous screenshot.

At the bottom of the screen, a section allows to combine the search steps. 

Only the boolean operators can be used : AND, OR, NOT. Proximity operators will give unrelevant hits.

For example, in the displayed case, a first query (Step 1) containing keywords specific to the topic of interest is combined (with an OR) to a larger keyword search (Step 2) which has been limited to the technical domain related to "transport" (step 3) in order to avoid noise.

Once satisfied by the combination, the resulting documents can be reviewed by clicking on the Action button "Show results".

Up to 200 steps can be created in the search history. Any step can be removed manually by clicking the "delete" action the right side of the screen. A button to erase all the search history is also available on the top left end corner of the interface.