Description:

• The variable error represents an input during the evaluation of which a type or execution error has been detected or is to be detected. Inputs that are syntactically correct but wrongly typed evaluate to error at some stage. Inputs that are correctly typed but containing commands that depend on side-effects that cannot be performed or inputs that are wrongly typed at meta-level (cf. parse), evaluate to error.
  
  Remark that in contrast to all other elements of the Sollya language, error compares neither equal nor unequal to itself. This provides a means of detecting syntax errors inside the Sollya language itself without introducing issues of two different wrongly typed inputs being equal.