MoboMath works only with valid mathematical expressions, but sometimes intermediate steps in an editing process result in invalid expressions. In these cases, MoboMath will insert a placeholder for you to fill in later.
For example, if you cross out the denominator in a fraction, MoboMath adds a placeholder where the missing denominator should be.
You can then use ordinary editing techniques to replace the placeholder with a valid subexpression – for example, you can write on top of the placeholder, or drag something onto the placeholder.