Ask the person working a run which way they are working the run and if it is alright to step in below them or if they would prefer you to move down river.

Like this is going to happen. I hate it when somebody (usually a gear guy) steps in right below me and grows roots and won't move as I work down river. I don't mind if they move at the same rate as me.
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They say that the man that gets a Ph.D. is the smart one. But I think that the man that learns how to get paid to fish is the smarter one.