fn check_consecutive_binary_ops(seq: &[TokenTree]) -> Result<(), Error>Expand description
Returns an error if two consecutive non-unary binary operators appear
with nothing between them (x * * y, x / / y).
Unary-capable operators (+, -, !, ~, ?) are never flagged —
they chain legally (x - - y). Joint-spaced pairs are never flagged
either: that skips every multi-char operator (<-, ->, <=, %%,
%in%’s delimiters) and **, which R’s parser accepts as an
undocumented synonym for ^.