Unsupported colon operator with char operands

[Lokinyx]

E = 'z':-1:'a'

Error:
For colon operator with char operands, first and last operands must be char.

while
E = zyxwvutsrqponmlkjihgfedcba
is expected

[Nelson-numerical-software]

'z':single(-1):'a' should return an error (too)