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In a 9x9 grid (81 cels), each symbol/color is represented with no less than 6 and no more than 7 L-shapes.
If an L shape, or even a single cell is surrounded by 3 L's with different symbols, you know for sure that the 4th symbol goes here.
Look for such combinations, as they are very helpful!
Each grid has only a single solution. If a configuration has two possibilities it is suspect.
An example of this an empty L-shape, that has no pre-filled cels and that is entirely surrounded with only two different symbols. Such an L-shape could then be either of the remaining symbols. This situation should not occur - when it does, something has likely gone wrong before.
Or perhaps two pieces can seemingly be interchanged freely. A neighboring piece will need to be placed in order to define the single right possibility – and sometimes, you can even use this knowledge to find another L-shape.