Extend P(A|B) to more vars

[xiaoyingpu]

• Usability: do users understand P(A, B | C, D) even if our syntax supports it?
• Probably need to manipulate the syntax tree to override operator precedence
• P() loses track of the original data types when normalizing the dataframe
• Currently, P(A | B) implies P(A | B == TRUE)
  • Make this more explicit and extend it to P(A | B == 'c')