Notes:

• You can type special characters with a backslash prefix, as shown by hovering over the character bar above. Documentation links for primitives are here.
• Any modifier with enter will execute (not just shift).
• For errors in dyadic functions or modifiers, the caret indicating position will point to the left argument or operand, not the function itself.
• Support for function headers and multiple bodies is missing, and Under (⌾) only works for a small class of structural functions. Fill elements aren't tracked: instead 0 is always used. Performance is bad.