I can't tell how to use the defining forms at https://calculist.io/markup. These work:

    * \= 2*3
    * \(x, y) = x+y

However I'm not sure how to define a new function. Here's what I tried:

    * \: foo (x, y) = x+y  # no error
    * \=foo(2, 3)  # error

    * \:barr = 2*3  # no error
    * \=barr  # error