sub linear-combination(@coeff) {
    (@coeff Z=> map { "e($_)" }, 1 .. *)
    .grep(+*.key)
    .map({ .key ~ '*' ~ .value })
    .join(' + ')
    .subst('+ -', '- ', :g)
    .subst(/<|w>1\*/, '', :g)
        || '0'
}

say linear-combination($_) for
[1, 2, 3],
[0, 1, 2, 3],
[1, 0, 3, 4],
[1, 2, 0],
[0, 0, 0],
[0],
[1, 1, 1],
[-1, -1, -1],
[-1, -2, 0, -3],
[-1 ]
;