Definitions should properly tailor English

[niazim3]

Issue #740 will soon be closed after altering a definition of an instance model's symbol to one that doesn't conflict with how symbols are hard-coded to be described using "is the".

@JacquesCarette mentioned that "an even better solution would need us to have more information about the structure of Definitions to properly tailor the English. Which, long term, is the right thing to do."

[cd155]

#2802 is an example of this issue. In the single and double pendulum examples, two SRSs are contained almost the same meaning, but the grammar requires the use of singular or plural in sentences.