Etymology
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protero- 
before vowels proter-, word-forming element meaning "former, earlier," from Greek proteros "before, former, anterior," from PIE *pro-, from root *per- (1) "forward" (hence "before, first").
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at- 
assimilated form of ad- "to, toward, before" before stems beginning in -t-; see ad-. In Old French and Middle English regularly reduced to a-, later restored.
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preter- 

also praeter-, word-forming element meaning "beyond; over, more than in quantity or degree," from Latin praeter (adverb and preposition) "beyond, before, above, more than," properly comparative of prae "before," from PIE root *per- (1) "forward," hence "in front of, before."

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of- 
assimilated form of ob- before -f-.
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op- 
assimilated form of ob- before -p-.
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sus- 
assimilated form of sub- before -s-.
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dif- 

assimilated form of dis- before -f-.

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sum- 
assimilated form of sub- before -m-.
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sur- (2)
assimilated form of sub- before -r-.
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