Responsa for Yevamot 209:2

מני אילימא רבי אליעזר חלצה ולא רקקה ולא קראה חליצתה כשירה והא אמר רבי אליעזר (דברים כה, ט) ככה יעשה דבר שהוא מעשה מעכב אלא פשיטא רבי עקיבא וקתני רקקה ולא חלצה ולא קראה חליצתה פסולה למאן

if she recited<span class="x" onmousemove="('comment',' V. p. 721, n. 14. ');">²</span> but did not spit nor draw off the shoe, there is here no reason whatsoever for apprehension.<span class="x" onmousemove="('comment',' I.e., even levirate marriage is permitted. ');">⁴</span> Now, whose [view is here represented]? If it be suggested [it is that of] R. Eliezer, [how could it be stated that] 'if she drew off [the levir's shoe] but did not spit nor recite, her <i>halizah</i> is valid' when, surely, R. Eliezer said: SO SHALL BE DONE, ANYTHING WHICH IS A DEED IS A SINE QUA NON? It is consequently obvious [that it is the view of] R. Akiba; and yet it was stated that 'if she spat but did not draw off the shoe nor recite, her <i>halizah</i> is invalid'. To whom, [however, does the invalidity cause her to be forbidden]?<span class="x" onmousemove="('comment',' The expression vjuxp, here rendered 'invalid', bears in the original a double meaning: (a) the halizah itself is invalid and (b) the woman becomes invalid, i.e., unfit to contract a marriage. V. infra note 8. ');">⁵</span>

Teshuvot Maharam