Interprete `√` as `sqrt`

Could you make it so that √ is interpreted the same as sqrt?

[icasdri]

Quick workaround: type √(x) = sqrt(x). This defines a new function √ that just wraps the square root. But as Mather doesn't currently save state per #7, this will only last for this session.

Some solutions from here:

• The third part of Multiple keyboard enhancements #14 addresses some part of this, but that only changes the display on the keyboard not an actual function
• We could allow users to globally define custom functions that persist, so in addition to √(x) = sqrt(x) they coud also permanently add myFunc(x) = 42 * x or the like.
• We could should include a selection of function aliases by default (i.e. special-casing certain symbols like √)