Given a starting index, return the index of the next token on channel. Return {@code i} if {@code tokensi} is on channel. Return the index of the EOF token if there are no tokens on channel between {@code i} and EOF.
See Implementation
Given a starting index, return the index of the next token on channel. Return {@code i} if {@code tokensi} is on channel. Return the index of the EOF token if there are no tokens on channel between {@code i} and EOF.